Make time pattern validation less strict

[soininen]

Equal time interval end points can now be combined, e.g. M8-8 can be replaced by M8.

Resolves spine-tools/Spine-Toolbox#2141

Checklist before merging

• Documentation (also in Toolbox repo) is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.66%. Comparing base (b416c16) to head (64daa53).

Files with missing lines	Patch %	Lines
spinedb_api/parameter_value.py	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #602      +/-   ##
==========================================
- Coverage   85.67%   85.66%   -0.01%     
==========================================
  Files          81       81              
  Lines       11022    11029       +7     
  Branches     1622     1623       +1     
==========================================
+ Hits         9443     9448       +5     
- Misses       1222     1224       +2     
  Partials      357      357              

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