[Cython] Remove Py3.4 support in Cython 3.0?

[Stefan Behnel]

Hi,

I looked up where Python 3.4 is still being used.

Ubuntu 14.04 – EOS 2019, EOL April 2022
Debian 8 – EOL
Centos 6 – EOL

Later distribution versions have newer CPythons:

Ubuntu 16.04: Py3.5  (EOS April 2021, EOL 2024)
Debian 9: Py3.5  (EOL June 2022)
Centos 7: Py3.6  (EOL June 2024)

I think we can safely drop Py3.4 support already for Cython 3.0.

The advantage isn't big, since we still support Py2.7, but we can at least 
stop testing it, and start relying on the Py3.5 C-API for Py3.

Given that Py3.5 is also going out of support in major Linux distributions 
within a year's time, I'd suggest we drop support for it in Cython 3.1, 
together with Py2 support, making the minimum version Py3.6. That would 
mean that we can start using f-strings, dict ordering, and what not.

Any objections?

Stefan