Handle the port forward being cancelled

[jacobtomlinson]

It's ok for serve_forever to be cancelled.

Also added a little sleep to handle calling pf.start() and pf.stop() in quick succession.

Closes #221

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (0280954) 94.99% compared to head (c28aca1) 94.76%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
kr8s/portforward.py	50.00%	5 Missing ⚠️
kr8s/_portforward.py	33.33%	2 Missing ⚠️
kr8s/tests/test_gen.py	90.47%	2 Missing ⚠️
kr8s/_objects.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
- Coverage   94.99%   94.76%   -0.23%     
==========================================
  Files          26       27       +1     
  Lines        2579     2638      +59     
==========================================
+ Hits         2450     2500      +50     
- Misses        129      138       +9     

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