ModuleNotFound

[fossildoc]

Describe the bug
Any type of pip install, including --upgrade of pip itself, always results in the same error:
ModuleNotFoundError: No module named 'pip._internal.vcs.mercurial'

Here is the entire output when I tried to run pip install pudb. Notice it uses python3.9 site packages, but if I run it with python3.8 -m pip install pudb, I get exactly the same thing, but with python3.8 site packages.

Traceback (most recent call last):
  File "/home/richard/.local/bin/pip", line 8, in <module>
    sys.exit(main())
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/cli/main.py", line 68, in main
    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/commands/__init__.py", line 109, in create_command
    module = importlib.import_module(module_path)
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 850, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/commands/install.py", line 14, in <module>
    from pip._internal.cli.req_command import (
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/cli/req_command.py", line 20, in <module>
    from pip._internal.index.collector import LinkCollector
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/index/collector.py", line 42, in <module>
    from pip._internal.network.session import PipSession
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/network/session.py", line 31, in <module>
    from pip._internal.network.auth import MultiDomainBasicAuth
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/network/auth.py", line 22, in <module>
    from pip._internal.vcs.versioncontrol import AuthInfo
  File "/home/richard/.local/lib/python3.9/site-packages/pip/_internal/vcs/__init__.py", line 7, in <module>
    import pip._internal.vcs.mercurial
ModuleNotFoundError: No module named 'pip._internal.vcs.mercurial'

My Platform
Ubuntu 20.04

[PythonCoderAS]

How did you install pip?

[DiddiLeija]

This might be related to #5599, but I'm not sure at all. If not, we need more information about the way you installed pip, as @PythonCoderAS suggested.

[fossildoc]

I install pip like this:
sudo apt update
sudo apt install python3-pip

[fossildoc]

I cannot install anything at all using pip. I always get the mercurial message.

[uranusjr]

If you use apt you should report to your OS provider (e.g. Ubuntu).