tools/release-packages: Remove the ability to push changes.

This commit removes the following:

* This script's dependency on gitpython, it is not maintained actively.
* The ability to push changes in this and the main zulip/ repo. Doing
  so automatically was turning out to be cumbersome and buggy and
  doing it manually doesn't take significantly longer anyway.
* The ability to automatically increment PROVISION_VERSION in
  zulip/version.py. Again, this was too buggy and it doesn't take
  much longer to just increment it by hand.

Removing the above features made the script a lot easier to maintain
and read.

requirements.txt

@@ -1,6 +1,5 @@
 crayons
 twine
-gitpython
 coverage>=4.4.1
 pycodestyle==2.3.1
 -e ./zulip