[RavSinghChandan]

read_object is annotated Union[PdfObject, int, str, ContentStream], but it never
returns an int or a str. Every branch hands back a PdfObject subclass - NameObject,
DictionaryObject, ArrayObject, BooleanObject, NullObject, NumberObject or
IndirectObject - and ContentStream is a DecodedStreamObject, so it is already a
PdfObject too.

Because the annotation was wider than the truth, the callers had to work around it.
_get_object_from_stream repeated the same union, and get_object carried two
type: ignore[assignment] comments plus two cast(PdfObject, retval) calls that existed
only to undo the phantom types again before decrypt_object. Narrowing the return type
lets all six go, including the last bare type: ignore left in the package.

Nothing changes at runtime, so I have not added tests. mypy reports the same 11
pre-existing errors before and after, all in _crypt_providers/_pycryptodome.py.

One thing worth flagging: I could not run the tests that download the corpus locally,
so those are down to CI. The offline suite passes, 1231 tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.04%. Comparing base (17d57a5) to head (a04176b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4038   +/-   ##
=======================================
  Coverage   98.03%   98.04%           
=======================================
  Files          57       57           
  Lines       11210    11208    -2     
  Branches     2110     2110           
=======================================
- Hits        10990    10989    -1     
+ Misses        123      122    -1     
  Partials       97       97           

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