Obsolescence Python 3.5 support plan

[EricZolf]

Hi,

from a past survey, we know that some of you still use Python 3.5, but
we'd like sooner or later to get rid of it and wanted to ask you if
there is a _big_ issue with it.

Here the reasons for the foreseen move:

- Python 3.5 is out-of-support by 13th of September according to [1]
- The oldest still supported Linux distro I know of (CentOS/RHEL 6) has
Python 3.6 available [2][3] (Debian Jessie is still supported but only
until June, see [4], and it anyway only has Python 3.4 available [5])
- Python 3.9 is on our front-door and will be released on October [1]
- with 4 supported Python versions we already need 30-40 minutes for our
merge pipeline, which is quite a lot, and I wouldn't want it to be any
longer, which would be the case with 5 supported Python versions.
- using PathLike could make the code slightly simpler [6], which is my
current priority.

So the plan would be to drop Python 3.5 support at the same time where
we add Python 3.9 support, which would be together with its availability
on Travis CI, already a given as `3.9-dev` [7], but I'd wait until the
rc is available (first tests look already good though).

So I envision something around summer/early fall, with a last stable
version supporting 3.5 before we switch Python versions.

Any comments on those plans?

KR, Eric

[1] https://devguide.python.org/#status-of-python-branches
[2]
https://access.redhat.com/documentation/en-us/red_hat_software_collections/3/html/3.4_release_notes/chap-rhscl#sect-RHSCL-Features
[3] https://www.softwarecollections.org/en/scls/rhscl/rh-python36/
[4] https://www.debian.org/releases/jessie/
[5] https://packages.debian.org/jessie/python3
[6] https://docs.python.org/3.8/library/os.html#os.PathLike
[7] https://docs.travis-ci.com/user/languages/python/#python-versions