Remove support for Python 3.5

[PhilippWendler]

BenchExec currently supports Python 3.5 or newer and at some point we will raise the required Python version to 3.6. The official support for Python 3.5 has ended already.

The last version of Ubuntu that has only Python 3.5 is Ubuntu 16.04, which will go out of LTS support in 2021-04, so April 2021 would be a good point for BenchExec to drop Python 3.5.

Debian 9 also provides only Python 3.5, but it will have LTS support until 2022-06, which I prefer to not wait for because Python 3.6 brings quite some nice improvements (better type annotations, format strings, and better path support).

So please comment if removing support for Python 3.5 in 2021-04 would be a problem for you.

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For further Python versions, we will also drop support for them step-by-step. For Python 3.6, this is an open question. End of official support is already on 2021-12-31 and Debian never shipped it in a release, but Ubuntu 18.04 has it and still has LTS support for quite some time until 2023-04. On the other hand, there are official packages for Python 3.8 in the package repositories for Ubuntu 18.04. So we will consider removing Python 3.6 in beginning of 2022 and maybe Python 3.7 in 2023. Again, please comment if this is a problem for you or if you have other suggestions.