Drop Python 3.7, fix genre shenanigans, parse 'with' and 'w/'

(#52) genre: do not fail parsing a release without any keywords, for example https://amniote-editions.bandcamp.com/album/ae-mj-0011-the-collective-capsule-vol-1
(#54) Ensure that our genre matching rules also apply to genres delimited by a dash, not only space.

album:
- handle some edge cases when string EP or LP is followed with data relevant to the album
- do not remove artist or label when it is preceded by x or followed by characters ', _ and &, or words EP, LP and deluxe
- handle apostrophes more reliably
- Do not remove VA or V/A from the beginning when followed by a word or a number
album / title:
- Remove Various - from album and track names
- Handle this album sent to us by the devil himself
catalognum:
- allow catalogue numbers like Dystopian LP01
- parse a range of catalogue numbers when it is present, for example TFT013SR - TFT-016SR
comments: use value None when there are no comments. In contrast to returning an empty string, this way during beets import the previous comment on the track will be kept if the Bandcamp release does not have a description.
label: label is now correctly obtained for single releases when it is available.
title:
- consider with and w/ as markers for collaborating artists
- remove bonus - - Artist - Title (bonus - something) -> Artist - Title (something)

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
beetsplug/bandcamp/album.py	52	54	96.3%
beetsplug/bandcamp/track_names.py	81	83	97.59%
beetsplug/bandcamp/tracks.py	69	71	97.18%
beetsplug/bandcamp/init.py	42	45	93.33%
beetsplug/bandcamp/helpers.py	18	22	81.82%
<!--	Total:	340	353	96.32%	-->

snejus / beetcamp