Made writing custom slice sources easier

[forman]

• Slice items can now be a contextlib.AbstractContextManager so custom slice functions can now be used with @contextlib.contextmanager.
• Deprecated SliceSource.dispose() and introduced SliceSource.close().

To close #82, also see #84 and #86.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.93%. Comparing base (70e739c) to head (c9228ae).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #83   +/-   ##
=======================================
  Coverage   99.92%   99.93%           
=======================================
  Files          30       30           
  Lines        1428     1436    +8     
=======================================
+ Hits         1427     1435    +8     
  Misses          1        1           

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