Avoid importing contextlib in `_virtualenv`

[hauntsaninja]

This appears to be imported in a .pth file, so this file runs in basically every Python program in my environment. contextlib takes about 3ms to import and I'd prefer to not pay that cost if not strictly necessary.

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• [x] ran the linter to address style issues (tox -e fix)
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[hauntsaninja]

This was introduced in https://github.com/pypa/virtualenv/pull/2588 , presumably as part of a mass application of SIM105

[hauntsaninja]

Looks like tests are failing on #2685 as well...

[hauntsaninja]

(Also noob question: what is the os.path.join(__file__) idiom meant to do?)

[gaborbernat]

(Also noob question: what is the os.path.join(__file__) idiom meant to do?)

Probably a no-op, could try removing it.