Fixes some E275 - assert is a keyword. + other minor PEP8 fixes

[misl6]

Maintainer merge checklist

• [ ] Title is descriptive/clear for inclusion in release notes.
• [x] Applied a Component: xxx label.
• [ ] Applied the api-deprecation or api-break label.
• [ ] Applied the release-highlight label to be highlighted in release notes.
• [x] Added to the milestone version it was merged into.
• [ ] Unittests are included in PR.
• [ ] Properly documented, including versionadded, versionchanged as needed.

Like https://github.com/kivy/python-for-android/pull/2647:

• Assert is a keyword and a space is needed + the parentheses are not needed.
• A recent change in pycodestyle (See: https://github.com/PyCQA/pycodestyle/pull/1063) introduced this check.
• CI runs started to fail this recently. (See: https://github.com/kivy/kivy/runs/7679002182)

Additionally:

• Adds venv to flake8 excluded folders.
• Fixes an E501 issue in kivy/uix/behaviors/compoundselection.py