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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