Updated python version requested from the ci

[Iximiel]

Description

Python 3.7 has finally come to its EOL,I'm starting a series of PR for trying to see if python 3.9 can be used without to many modifications

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[GiovanniBussi]

Great!

Looks like with centos 7 this is not trivial.

However, I checked a previously running job and it looks like the breaking change was the switch to cython 3.0.

Maybe it's sufficient to specify cython<3 here?

[Iximiel]

Maybe it's sufficient to specify cython<3 here?

If it works I think it can be safely merged into 2.9 and then we should carefully update the master, because of the change in the CI

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.04% ⚠️

Comparison is base (0493ec1) 85.91% compared to head (0a6ce2b) 85.88%.

❗ Current head 0a6ce2b differs from pull request most recent head 163da37. Consider uploading reports for the commit 163da37 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             v2.8     #967      +/-   ##
==========================================
- Coverage   85.91%   85.88%   -0.04%     
==========================================
  Files         595      595              
  Lines       49731    49731              
==========================================
- Hits        42724    42709      -15     
- Misses       7007     7022      +15     

see 2 files with indirect coverage changes

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