Drop support for Python 3.7

[agriyakhetarpal]

Description

This change drops support for Python 3.7.

Fixes #2370

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

Base: 99.66% // Head: 99.66% // No change to project coverage 👍

Coverage data is based on head (222be25) compared to base (c89445e).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2379   +/-   ##
========================================
  Coverage    99.66%   99.66%           
========================================
  Files          269      269           
  Lines        20206    20206           
========================================
  Hits         20139    20139           
  Misses          67       67           

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[valentinsulzer]

Thanks @agriyakhetarpal !

[valentinsulzer]

@all-contributors add @agriyakhetarpal for infrastructure

[valentinsulzer]

@all-contributors add @agriyakhetarpal for infra