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Getting error "/usr/bin/python3.9: bad interpreter: No such file or directory" after installing pip in ubuntu 16.04

I installed python3.9 in ubuntu and also make it default which creates some problems. So I again make python 3.5 as default and removed python 3.9.
Then I tried to install pip by using command:
sudo apt-get install python3-pip
and it installed successfully but when I ran:
pip3 --version
it gives me this error:
/usr/bin/python3.9: bad interpreter: No such file or directory
I don't know why I removed python 3.9 completely with all its dependencies then why pip is not installing properly?
Please help me out here i'm really stuck.

This answer might help, you can run pip as a module, try python3 -m pip [commands] to see if it works.

Try running:
pip --version
It might work.

pip package installs okay but cannot be run

I am wanting to install a pip package on Debian but when I install it using
pip install bitmex-market-maker
or
pip install --user bitmex-market-maker
it doesn't work and gives me
-bash: marketmaker: command not found
error here
it works fine on my MacBook
I tried to reinstall reboot and even tried pip3 to no avail.

To run the libraries you first need to be inside the python environment.
At your terminal try:
python -m marketmaker

Python on anaconda cannot find azure.mgmt.datafactory

I am trying to run this tutorial
https://learn.microsoft.com/en-US/azure/data-factory/quickstart-create-data-factory-python
but I fail to install the packages. I tried several installations but I keep getting the error No module named 'azure.mgmt.datafactory' when trying to run from azure.mgmt.datafactory import DataFactoryManagementClient.
I am using anaconda and windows 10.
I tried running the recommended anaconda packages https://anaconda.org/anaconda/azure and https://anaconda.org/clinicalgraphics/azure-mgmt-resource under a python 3.5 environment and I also tried to manually install everything from github (https://github.com/Azure/azure-sdk-for-python) using
git clone git://github.com/Azure/azure-sdk-for-python.git
cd azure-sdk-for-python
python setup.py install
In both the normal (Python 3.6) and the new (Python 3.5, using Anaconda version with Python 3.5) environment. None of this worked.
What am I missing?
(Note that from azure.mgmt.resource import ResourceManagementClient worked fine with the anaconda installation)
EDIT
After the first response, I ran the following commands from the powershell
pip install azure-mgmt-resource
pip install azure-mgmt-datafactory
pip install azure-mgmt
which resulted in ModuleNotFoundError: No module named 'azure.mgmt'
Uninstalling the three packages and installing azure-mgmt as a first one did not solve the issue either. However, I don't know how to uninstall the manually installed package from python setup.py install, which still might be an issue.

Have you tried pip install in powershell/cmd?
pip install azure-mgmt-datafactory
Update (Jan's answer):
pip freeze > requirements.txt
pip uninstall -r requirements.txt
python -m pip install azure-common
python -m pip install azure-mgmt
python -m pip install azure-mgmt-datafactory (this might not be needed as it comes with azure-mgmt)

Ok, this is how I got the required azure libraries to work (thx to Saul Cruy, who gave me the idea)
Using this post What is the easiest way to remove all packages installed by pip?, I created a requirements file in PowerShell
pip freeze > requirements.txt
In this file, I manually kept only the entries with azure.
Then, I deleted all packages in the file
pip uninstall -r requirements.txt
The steps above were repeated twice, as upon first delete, some azure packages survived.
Then, I ran (all in PowerShell, in that order)
python -m pip install azure-common
python -m pip install azure-mgmt
python -m pip install azure-mgmt-datafactory
The reason might(!) be that installing packages in the anaconda console using the conda commands causes confusion in the dependencies (I tried a similar approach in a conda environment as it seemed like a good idea to seperate the azure packages from the other ones, but without success).

Python pip installation in ubuntu

I am new to Ubuntu and was trying to install pip using get-pip.py , when I received this message.
Requirement already up-to-date: pip in /usr/local/lib/python2.7/dist- packages/pip-8.1.2-py2.7.egg
But when I enter pip -V, I receive an error saying :
The 'pip==7.1.0' distribution was not found and is required by the application
Complete error
I was trying to install new packages using pip install <packagename> but this command gives the same error as previous .

I suggest installing pip using the package manager. Open up a terminal and enter
sudo apt-get install python-pip
That should install the pip ubunutu package.

You should not be installing pip for the default python installation, neither from the package manager nor using get-pip.py. So you can never use it to break the system python.
Instead always use virtualenv (created from the default/system python or from a newer version), activate and use pip in that environment.

Uninstall Python package using pip3

I am working with Python 3.4 and have installed a package (spyder) using pip3 install. It is works as it should and I can start it from the terminal.
Since I want to switch my IDE, I tried to uninstall it using pip3 uninstall in the same way as the installation. But this gives me a message that no files can be found.
Here's my terminal output:
cord#laptop:~$ sudo pip3 freeze
Coopr==3.5.8748
...
spyder==2.3.4
...
xdiagnose==3.6.3build2
xkit==0.0.0
cord#laptop:~$ sudo pip3 uninstall spyder
Can't uninstall 'spyder'. No files were found to uninstall.
My OS is Ubuntu 14.04 x64.
Any hints?

Go to /usr/local/bin and find the packages you want to delete
Then just use sudo rm -r spyder or whatever the directory name is.
That way the files get manually removed from your system.