python_requirement: fix ENV for python3
python_requirement: fix ENV for python3

Closes Homebrew/homebrew#43453.

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