Improve typing in Session.get_pilot_managers

[eirrgang]

• Improve handling for optional missing pmgr_uids.
• Remove unnecessary restriction on iterable type.
• Correct the return type documentation.

[codecov[bot]]

Codecov Report

Merging #2968 (02dd83f) into devel (b1a6002) will not change coverage. The diff coverage is 0.00%.

:exclamation: Current head 02dd83f differs from pull request most recent head 42275c5. Consider uploading reports for the commit 42275c5 to get more accurate results

@@           Coverage Diff           @@
##            devel    #2968   +/-   ##
=======================================
  Coverage   41.63%   41.63%           
=======================================
  Files          95       95           
  Lines       10497    10497           
=======================================
  Hits         4370     4370           
  Misses       6127     6127           

Impacted Files	Coverage Δ
src/radical/pilot/session.py	54.02% <0.00%> (ø)

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