Python annotations and pylint cleansing

[Stuart Prescott]

Hi all,

Please find code I would like to merge into python-debian at:

https://salsa.debian.org/python-debian-team/python-debian/merge_requests/5

This MR includes 

* type annotations (compatible with both Python 2 and 3) for the module
* changes needed to fix bugs highlighted by the type annotations
* changes needed to fix bugs highlighted by pylint
* a pylint config to silence more annoying warnings that we are unlikely to 
fix

The final code is clean with mypy (type checking) and pylint3 (current 
upstream release, since the Debian package is not currently installable).

I might enable mypy and pylint as part of the CI on salsa so that we can 
continue to polish the code and ensure that this current status continues.

Comments welcome! I'd like to merge this branch in the next few days -- it 
touches almost every file in the package many many places meaning that it is 
impossible to do anything else on python-debian while this MR remains 
unmerged.

cheers
Stuart

-- 
Stuart Prescott    http://www.nanonanonano.net/   stuart at nanonanonano.net
Debian Developer   http://www.debian.org/         stuart at debian.org
GPG fingerprint    90E2 D2C1 AD14 6A1B 7EBB 891D BBC1 7EBB 1396 F2F7