Change LICENSE to GPLv3+, after discussion with intrigeri.

 * ADD docs/change-license-emails.txt, which includes email exchanges between
   myself and intrigeri, including links to Debian and LEAP mailing lists with
   arguments for and against using AGPL for a library.
 * CHANGE license header for all files.
 * CHANGE LICENSE file and gnupg/copyright.py to use GPLv3+ text.
Update docs/index.rst to point to online text of new license.
Remove TODO item for keyring separation in GPG.gen_key_input() and GPG.gen_key().
Add scripts/pep8.py for checking PEP conformity.