python_dependency: fixes, features, cleanup.
python_dependency: fixes, features, cleanup.

- PythonDependency now implies Python 2.7
- PythonDependency now uses brewed Python for bottling
- Use double-quotes everywhere

Closes Homebrew/homebrew#27112.

aw1621107/brew,claui/brew,nandub/brew,hanxue/linuxbrew,maxim-belkin/brew,Homebrew/brew,JCount/brew,vitorgalvao/brew,amar-laksh/brew_sudo,pseudocody/brew,vitorgalvao/brew,mahori/brew,rwhogg/brew,ilovezfs/brew,nandub/brew,claui/brew,gregory-nisbet/brew,staticfloat/brew,JCount/brew,Linuxbrew/brew,bfontaine/brew,JCount/brew,AnastasiaSulyagina/brew,toonetown/homebrew,reitermarkus/brew,nandub/brew,konqui/brew,ilovezfs/brew,reelsense/brew,palxex/brew,staticfloat/brew,mahori/brew,amar-laksh/brew_sudo,EricFromCanada/brew,AnastasiaSulyagina/brew,gregory-nisbet/brew,mahori/brew,jmsundar/brew,gregory-nisbet/brew,muellermartin/dist,gordonmcshane/homebrew,nandub/brew,konqui/brew,tonyg/homebrew,konqui/brew,sjackman/homebrew,jmsundar/brew,jmsundar/brew,EricFromCanada/brew,reitermarkus/brew,EricFromCanada/brew,pseudocody/brew,reelsense/brew,aw1621107/brew,bfontaine/brew,gordonmcshane/homebrew,tonyg/homebrew,alyssais/brew,gordonmcshane/homebrew,Linuxbrew/brew,mistydemeo/homebrew,alyssais/brew,EricFromCanada/brew,Homebrew/brew,DomT4/brew,Homebrew/brew,MikeMcQuaid/brew,hanxue/linuxbrew,zmwangx/brew,zmwangx/brew,sjackman/homebrew,maxim-belkin/brew,mistydemeo/homebrew,staticfloat/brew,Homebrew/brew,mgrimes/brew,reitermarkus/brew,ilovezfs/brew,sjackman/homebrew,rwhogg/brew,tonyg/homebrew,aw1621107/brew,vitorgalvao/brew,MikeMcQuaid/brew,muellermartin/dist,mistydemeo/homebrew,reitermarkus/brew,mgrimes/brew,claui/brew,DomT4/brew,rwhogg/brew,konqui/brew,mahori/brew,MikeMcQuaid/brew,palxex/brew,DomT4/brew,JCount/brew,toonetown/homebrew,hanxue/linuxbrew,MikeMcQuaid/brew,Linuxbrew/brew,AnastasiaSulyagina/brew,reelsense/brew,alyssais/brew,toonetown/homebrew,muellermartin/dist,bfontaine/brew,DomT4/brew,vitorgalvao/brew,pseudocody/brew,zmwangx/brew,claui/brew,sjackman/homebrew,maxim-belkin/brew,mgrimes/brew,Linuxbrew/brew,amar-laksh/brew_sudo,palxex/brew