Remove explicit support for Python 3.4 and 3.5, adding tests for python 3.9

[pietermarsman]

Pull request

Closes #503

Python 3.4 and 3.5 are both end-of-life. No more releases are made.

Since the python software foundatation does not longer support them, we could also remove explicit support for them. This would allow us to:

• add type annotations (not possible in python 3.4).

How Has This Been Tested?

Updated travis and tox configuration.

Checklist

• I have added tests that prove my fix is effective or that my feature
  works
• I have added docstrings to newly created methods and classes
• I have optimized the code at least one time after creating the initial
  version
• I have updated the README.md or I am verified that this
  is not necessary
• I have updated the readthedocs documentation or I
  verified that this is not necessary
• I have added a consice human-readable description of the change to
  CHANGELOG.md

[pietermarsman]

I updated more than one line in the CHANGELOG.md, because I noticed that lots of them reference the issue instead of the PR. Now they all map to a PR.