Deprecate Python 3.4 and 3.5

[estshorter]

Pull request

For issue #503
This code is based on 3fbaa57.

How Has This Been Tested?

I tested in my environment.

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

[Ev2geny]

Hello, I think also this line needs to be updated

pdfminer.six/tox.ini

Line 2 in f03657e

envlist = py{34,35,36,37,38}

[estshorter]

I think tests should be removed after dropping support.

[flywire]

Why deprecate Python 3.4 and 3.5? What were there issues?

[estshorter]

Did you see #503 ?
Because it takes effort to support ancient Python (3.5 and earlier).
For example, we cannot use useful syntax like f-string in those versions.

[flywire]

OK, suggest add End of support comment: https://www.python.org/downloads/