Add quarterly pruning strategy.

[cr1901]

This implements and would close #4750 as described. It works for my use case, but is not ready to be merged due to no tests and docs, and there are subtleties to be addressed wrt what a quarter year actually means.

To do:

• Split into --keep-3monthly (using the same logic --keep-monthly does) and --keep-13weekly (using the ISO 8601 week-based year), maybe get rid of the latter entirely.
• Tests
• Docs

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.50%. Comparing base (adf2d8c) to head (0e07ab4).
Report is 8 commits behind head on 1.4-maint.

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Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #8337      +/-   ##
=============================================
- Coverage      82.69%   82.50%   -0.20%     
=============================================
  Files             38       38              
  Lines          11045    11070      +25     
  Branches        2122     2128       +6     
=============================================
- Hits            9134     9133       -1     
- Misses          1345     1368      +23     
- Partials         566      569       +3     

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[ThomasWaldmann]

Extending the tests would be good, so this code gets executed and tested.

[cr1901]

Feedback addressed, tests are coming along and should be done soon (30 mins?).

[cr1901]

@ThomasWaldmann Okay to merge if you don't have additional changes requested.

[ThomasWaldmann]

Same here: port to master required after merge, port to 1.2-maint is very optional, considering this is new functionality and 1.2-maint will be phased out in a while.

[ThomasWaldmann]

@cr1901 Thanks!

[ThomasWaldmann]

I'll do the master port now.