python: handle env filtering on pth check
python: handle env filtering on pth check

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.

DomT4/brew,Linuxbrew/brew,MikeMcQuaid/brew,claui/brew,ilovezfs/brew,DomT4/brew,JCount/brew,Homebrew/brew,reelsense/brew,claui/brew,Linuxbrew/brew,mahori/brew,vitorgalvao/brew,nandub/brew,MikeMcQuaid/brew,aw1621107/brew,alyssais/brew,DomT4/brew,reelsense/brew,MikeMcQuaid/brew,nandub/brew,vitorgalvao/brew,EricFromCanada/brew,mgrimes/brew,Linuxbrew/brew,maxim-belkin/brew,konqui/brew,reitermarkus/brew,konqui/brew,konqui/brew,Homebrew/brew,claui/brew,nandub/brew,aw1621107/brew,JCount/brew,mgrimes/brew,hanxue/linuxbrew,JCount/brew,alyssais/brew,MikeMcQuaid/brew,mahori/brew,claui/brew,ilovezfs/brew,maxim-belkin/brew,konqui/brew,EricFromCanada/brew,aw1621107/brew,pseudocody/brew,alyssais/brew,reitermarkus/brew,nandub/brew,Homebrew/brew,vitorgalvao/brew,vitorgalvao/brew,reitermarkus/brew,pseudocody/brew,mgrimes/brew,sjackman/homebrew,sjackman/homebrew,hanxue/linuxbrew,ilovezfs/brew,JCount/brew,sjackman/homebrew,DomT4/brew,hanxue/linuxbrew,sjackman/homebrew,Linuxbrew/brew,Homebrew/brew,reitermarkus/brew,mahori/brew,EricFromCanada/brew,maxim-belkin/brew,reelsense/brew,EricFromCanada/brew,mahori/brew,pseudocody/brew