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 had
been used at one time, the last reference to it appears to have
been removed in 4545762 (gitpython-developers#1295).

This doesn't emit a DeprecationWarning when those attributes are
accessed in the git.compat module. (That might be valuable thing to
do in the future, if the git.compat module is to remain
non-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>.)