Fix isort failures in metrics

[arnaudgelas]

Remove from skipped module in pyproject.toml and fix failures on:

• pytorch_lightning/metrics/*.py

What does this PR do?

Relates to #4805

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[Borda]

seems you fix introduced some cyclic import, mind check...

pytorch_lightning/metrics/classification/average_precision.py:18: in <module>
    from pytorch_lightning.metrics import Metric
E   ImportError: cannot import name 'Metric'

[codecov]

Codecov Report

Merging #5528 (73cbbc3) into release/1.2-dev (c8f605e) will increase coverage by 0%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release/1.2-dev   #5528   +/-   ##
===============================================
  Coverage               93%     93%           
===============================================
  Files                  153     152    -1     
  Lines                10803   10741   -62     
===============================================
- Hits                  9999    9953   -46     
+ Misses                 804     788   -16