Change env python to env python3

[porglezomp]

I don't see any of the other PRs doing a full replacement of all the shebangs—this just changes all of them without doing any of the work to make sure that's fine.

[shahmishal]

We should drop __future__

[porglezomp]

this is intended as just dropping all the shebangs and not doing any compatibility cleanups, just making it easy to test if anything breaks with Python 3 (since all the __future__ etc. should keep working)

[porglezomp]

Probably shouldn't be merged at all

[Robertorosmaninho]

I suggest you erase this line too since there is no need for python 2.7 after these modifications.

In the case of macOS 12.3 that Python2.7 isn't in /usr/bin this line raises the following error:

xcrun: error: sh -c '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -sdk macosx -find python2.7 2> /dev/null' failed with exit code 17664: (null) (errno=No such file or directory)
xcrun: error: unable to find utility "python2.7", not a developer tool or in PATH
xcrun: error: sh -c '/Applications/Xcode.app/Contents/Developer/usr/bin/xcodebuild -sdk macosx -find python2.7 2> /dev/null' failed with exit code 17664: (null) (errno=No such file or directory)
xcrun: error: unable to find utility "python2.7", not a developer tool or in PATH

[porglezomp]

Going to just run some of the CI jobs on this

[porglezomp]

@swift-ci please test

[porglezomp]

@swift-ci Please Python lint

[porglezomp]

@swift-ci Please smoke benchmark