Start testing on Python 3.9 and stop on EOL Py3.5

[cclauss]

• https://devguide.python.org/devcycle/#end-of-life-branches
• https://devguide.python.org/#status-of-python-branches

[codecov-io]

Codecov Report

Merging #682 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #682   +/-   ##
=======================================
  Coverage   58.24%   58.24%           
=======================================
  Files          23       23           
  Lines        4273     4273           
  Branches      628      628           
=======================================
  Hits         2489     2489           
  Misses       1605     1605           
  Partials      179      179           

Flag	Coverage Δ
#py35	58.22% <ø> (ø)
#py36	58.27% <ø> (ø)
#py37	58.27% <ø> (ø)
#py38	58.28% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 772f5b3...604ead1. Read the comment docs.

[iredmail]

Debian 9 ships 3.5 and it’s still widely deployed, I suggest keeping 3.5 support for at least 1-2 years.

[iredmail]

@cclauss really drop 3.5? I hope to keep it for 1-2 years.

[cclauss]

I was hoping that we could move to f-strings ;-)

[iredmail]

README.md should update version number to 0.62 or 0.7.