Python 3 only, breaking backwards compatibility

A considerable amount of refactoring occurred for this build, chiefly to get
rid of Python 2.7 support. While the functionality did not change much for this
version, we have switched from a pkgutil-style pydrobert namespace to
PEP-420-style namespaces. As a result, this package is not
backwards-compatible with previous pydrobert packages! Make sure that if any
of the following are installed, they exceed the following version thresholds:

• pydrobert-param >0.2.0
• pydrobert-kaldi >0.5.3
• pydrobert-pytorch >0.2.1

Miscellaneous other stuff:

• Type hints everywhere
• Shifted python source to src/
• Black-formatted remaining source
• Removed future dependency
• Shifted most of the configuration to setup.cfg, leaving only a shell
  in setup.py to remain compatible with Conda builds
• Added pyproject.toml for PEP
  517.
• tox.ini for TOX testing
• Switched to AppVeyor for CI
• Messed around with documentation a little bit
• Added changelog :D