Add Typing

[willmcgugan]

It would be great to add typing information to Lomond, so we can use Mypy and other such tools.

[hukkin]

@willmcgugan Would you accept a PR dropping support for end-of-life Python versions (anything less than 3.6), to be able to use the 3.6 type annotation syntax?

Note that we can set python_requires>=3.6 metadata in setup.py so that pip will install a legacy lomond version (supporting legacy Python versions) if installing on Python version less than 3.6.