Fix the detection of an unhappy installation path in a venv under Windows

[tramora]

External tools like VSCode can modify the case of the path saved in sys.executable

Fixes #548

The case is indeed reproductible in saving the value of sys.executable and altering its case.

Implementing an automatic test would required extracting a function returning sys.executable and mocking it during the test.
Possible but but a bit overkill.

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TODO Before Asking for a Review

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  • Check that your changes render well in HTML: download the API Docs artifact and open index.html
  • If there are any problems it is faster to iterate by building locally the API Docs

[popescu-v]

See the comments.

[popescu-v]

See the remaining open comment.

[popescu-v]

A few remaining issues (see the comments).

[popescu-v]

LGTM. Change the sentence "This later sees case-insensitive paths." in the commit message to "The latter considers paths as case-insensitive." or another text along these lines.