mpv: Ensure that installing uses Python 3
mpv: Ensure that installing uses Python 3

- Over in Homebrew/linuxbrew-core, we thought we could remove
  `uses_from_macos "python@2"` given this looked to have macOS Python 3
  dependencies.
- Turns out, `bootstrap.py`'s shebang line had `/usr/bin/env python`,
  which catches "Python 2" on macOS but nothing on Linux.
- To make this actually run wholly with Python 3 everywhere, run
  `bootstrap` with python3 like the rest of the file does.

Closes #47586.

Signed-off-by: Rui Chen <5fd29470147430022ff146db88de16ee91dea376@gmail.com>

lembacon/homebrew-core,rwhogg/homebrew-core,lemzwerg/homebrew-core,jvillard/homebrew-core,sjackman/homebrew-core,makigumo/homebrew-core,mahori/homebrew-core,Moisan/homebrew-core,chrisfinazzo/homebrew-core,mvitz/homebrew-core,edporras/homebrew-core,jabenninghoff/homebrew-core,LinuxbrewTestBot/homebrew-core,aabdnn/homebrew-core,mfikes/homebrew-core,Linuxbrew/homebrew-core,filcab/homebrew-core,mvitz/homebrew-core,mvitz/homebrew-core,cblecker/homebrew-core,BrewTestBot/homebrew-core,Moisan/homebrew-core,adam-moss/homebrew-core,nbari/homebrew-core,stgraber/homebrew-core,sjackman/homebrew-core,adam-moss/homebrew-core,lemzwerg/homebrew-core,mfikes/homebrew-core,jabenninghoff/homebrew-core,mbcoguno/homebrew-core,jvillard/homebrew-core,Homebrew/homebrew-core,mbcoguno/homebrew-core,zyedidia/homebrew-core,uyjulian/homebrew-core,dsXLII/homebrew-core,edporras/homebrew-core,nandub/homebrew-core,makigumo/homebrew-core,dsXLII/homebrew-core,jvillard/homebrew-core,mahori/homebrew-core,lembacon/homebrew-core,aabdnn/homebrew-core,maxim-belkin/homebrew-core,stgraber/homebrew-core,chrisfinazzo/homebrew-core,nbari/homebrew-core,filcab/homebrew-core,BrewTestBot/homebrew-core,Homebrew/homebrew-core,Linuxbrew/homebrew-core,uyjulian/homebrew-core,JCount/homebrew-core,rwhogg/homebrew-core,zyedidia/homebrew-core,LinuxbrewTestBot/homebrew-core,nandub/homebrew-core,spaam/homebrew-core,mbcoguno/homebrew-core,cblecker/homebrew-core,spaam/homebrew-core,JCount/homebrew-core,maxim-belkin/homebrew-core,lembacon/homebrew-core