add type hints

[samuelcolvin]

Related issue number

fix #372, fix #366, fix #245, fix #280.

ref python/mypy#6251 and python/mypy#6239

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• HISTORY.rst has been updated
  • if this is the first change since a release, please add a new section
  • include the issue number or this pull request number #<number>
  • include your github username @<whomever>

[samuelcolvin]

should probably merge other PRs before this to avoid them conflicting.

[codecov]

Codecov Report

Merging #373 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #373   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          14     14           
  Lines        1934   1999   +65     
  Branches      375    375           
=====================================
+ Hits         1934   1999   +65