Fix pydocstyle D401: first line should be in imperative mood

[fridex]

Description of the changes being introduced by the pull request:

Make sure the codebase is more compliant with Python's docstring convention.

[coveralls]

Pull Request Test Coverage Report for Build 3724758599

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 98.195%

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Totals
Change from base Build 3711857675:	0.001%
Covered Lines:	1350
Relevant Lines:	1366

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💛 - Coveralls

[lukpueh]

See #2231 (review)

[jku]

I have no strong opinions on whether to have a linter for this but the fixes seem good and unlikely to be too annoying in git blame so I'm approving and merging these.