Fix cleanup of app binaries for venvs without pipx_metadata.json

[itsayellow]

• I have added an entry to docs/changelog.md

Summary of changes

Found this code bug with mypy --warn-unreachable.

This PR fixes a bug with pipx uninstall that would leave app binaries leftover in ~/.local/bin if the venv has no metadata file.

We were testing the wrong object to determine if metadata was present in uninstall.py. venv.pipx_metadata.main_package is in fact always not None, (it's always PackageInfo) so the else (for missing metadata case) was impossible to execute. Thus the special cleanup code for the non-metadata case to remove app binaries never executed.

I fixed it by changing the test for None to the object venv.pipx_metadata.main_package.package.

In addition, I updated venv.get_venv_metadata_for_package to add its currently-necessary second argument. (This code must have been unreachable for a while to not get flagged that it is missing an argument.)

Test plan

Tested by running

pipx install pycowsay
rm ~/.local/pipx/venvs/pycowsay/pipx_metadata.json
pipx uninstall pycowsay

This PR:

> ls -l ~/.local/bin/pycowsay
ls: /Users/mclapp/.local/bin/pycowsay: No such file or directory

pipx 0.16.1.0

> ls -l ~/.local/bin/pycowsay                     
lrwxr-xr-x  1 mclapp  staff  53 Mar 19 22:54 /Users/mclapp/.local/bin/pycowsay@ -> /Users/mclapp/.local/pipx/venvs/pycowsay/bin/pycowsay

[itsayellow]

I suppose I should be a "grown up" and add a test for this large block of code that isn't being tested...