Hardens mtime_cached_property to invalidate the cache for poor mtime resolutions

[tclose]

• Guesses the mtime resolution from the file system type (taking conservative estimates, e.g. ext4 -> 1 sec)
• Checks that enough time has passed since the last of the cached mtimes to be sure that no writes could have occurred between re-reads and the mtimes stay the same

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 6 lines in your changes missing coverage. Please review.

Project coverage is 92.34%. Comparing base (5b84d2b) to head (356ca38).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
fileformats/core/decorators.py	92.30%	3 Missing and 1 partial ⚠️
fileformats/image/raster.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   92.34%   92.34%   -0.01%     
==========================================
  Files          43       44       +1     
  Lines        5084     5108      +24     
  Branches      557      560       +3     
==========================================
+ Hits         4695     4717      +22     
- Misses        294      296       +2     
  Partials       95       95              

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