Porting to Python 3

As of version 1.4, beets supports both Python 3.x and 2.x. Sometime soon, we'll drop support for 2.x.

Coding conventions

calls to map() should be wrapped in list() when a list is expected as it returns an iter in python 3

PY3 only TODO

remove u' (and u") prefixes from code
remove u' (and u") prefixes from documentation
remove u' (and u") prefixes from wiki
delete https://github.com/beetbox/beets/wiki/String-literals page
run 2to3 to make sure we got everything
audit all calls to open() (or any other I/O) to see if we can read/write them as text vs binary
remove any PY2/3 version checks and the related code
search for all comments with Python 2,Python 3,Python2,Python3,PY2,PY3
remove py2 specific tests and packages in .travis.yml
remove any usage of six
remove # -*- coding: utf-8 -*- from all python files
replace buffer with memoryview
replace codecs.open, io.open with open
use explicit keyword arg names in function definitions - PY2: def foo(.., **kwargs) -> PY3: def foo(.., arg1=None, arg2=None, ...) as per https://docs.python.org/3.3/tutorial/controlflow.html#arbitrary-argument-lists
use str.format style ({}) in beets.logger or drop it completely https://docs.python.org/3/whatsnew/3.2.html#logging
Remove F405 from flake8 ignore configurtion if beets.logging is replaced
change flake8-coding configuration to no-accept-encodings
use more 3.x only functions from shutil to replace some of the functions in beets.util https://docs.python.org/3.4/library/shutil.html (TODO: be more specific)
drop mock dep since it's part of the stdlib (py >= 3.3)
remove enum34 dep since it's part of the stdlib ( >= 3.4)
Remove unnecessary traceback passing in HumanReadableException
integrate various suggestions from https://eev.ee/blog/2016/07/31/python-faq-why-should-i-use-python-3/
Switch windows CI to using github actions https://github.com/beetbox/beets/pull/3678