Python 3 only, breaking backwards compatibility with old pydrobert models

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-pytorch >0.2.1
• pydrobert-speech >0.1.0

Miscellaneous other changes include:

• Type hints everywhere
• Shifted python source to src/, alongside Kaldi source
• Updated numpy swig bindings for numpy 1.11.3
• Black-formatted remaining source
• Removed future and six, configparser
• Shifted a lot of the configuration to setup.cfg. There is still
  considerable work in setup.py due to the C extension
• Shifted documentation source from doc/ to docs/
• Shuffled around the indexing of documentation
• Added changelog :D