python: handle env filtering on pth check

[DomT4]

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Since ENV filtering became the default this has been chucking erroneous warnings from both brew doctor and things like brew install pygobject about needing to run:

echo 'import site; site.addsitedir("/usr/local/lib/python2.7/site-packages")' >> /Users/xyz/Library/Python/2.7/lib/python/site-packages/homebrew.pth

even though I had Homebrew's python installed.

[MikeMcQuaid]

Thanks again @DomT4!