Major changes:- Add a comment in the git.compat module above the definitions of  is_win, is_posix, and is_darwin stating that they are deprecated  and recommending that os.name (or, where applicable,  sys.platform) be used directly instead for clarity (and sometimes  accuracy).- Remove all uses of is_win and is_posix in git/ and test/,  replacing them with `os.name == "nt"` and `os.name == "posix"`,  respectively.There were no uses of is_darwin to be replaced. Although it hadbeen used at one time, the last reference to it appears to havebeen removed in4545762 (gitpython-developers#1295).This doesn't emit a DeprecationWarning when those attributes areaccessed in the git.compat module. (That might be valuable thing todo in the future, if the git.compat module is to remainnon-deprecated overall.)Two related less consequential changes are also included:- Improve ordering and grouping of imports, in modules where they  were already being changed as a result of no longer needing to  import is_<platform> (usually is_win) from git.compat.- Make minor revisions to a few comments, for readability. (This is  in addition to somewhat more substantial revisions of comments  where rewording was related to replacing uses of is_<platform>.)