Deprecate Python 3.7 support

[fabianegli]

It's finally time to deprecate Python 3.7.

Closes #1796

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

Merging #2210 (68e011b) into dev (c655a60) will increase coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##              dev    #2210      +/-   ##
==========================================
+ Coverage   71.83%   71.89%   +0.06%     
==========================================
  Files          77       77              
  Lines        8365     8355      -10     
==========================================
- Hits         6009     6007       -2     
+ Misses       2356     2348       -8     

Impacted Files	Coverage Δ
nf_core/subworkflows/test_yml_builder.py	15.48% <0.00%> (+0.31%)	⬆️
nf_core/modules/test_yml_builder.py	50.90% <100.00%> (+0.24%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.