Update python scripts from v2.x to v3.7+

[stephengaito]

In order to retire the use of python versions which no-longer have security patches, I have:

• Changed all instances of python2 to python.
• Updated print statements to be compatible with v3.7+
• Changed calls to next on FileIO streams (which no longer support next) to readline
• Added rstrip() to lines read from an FileIO stream so that double newlines were not introduced
  • these extra newlines were confusing the use of "block-mathematic-quotes" ('$$')
• Added an explicit text=True in the POpen call in git_version

The above changes were "tested" by building a local copy of the Stacks-project using make all

[aisejohan]

I have a question: after testing, did you check if the outcomes are the same as with the original version?