plistlib leaks ExpatError/LookupError instead of InvalidFileException for malformed XML

Bug description:

plistlib.load()/loads() is documented to raise plistlib.InvalidFileException when a file cannot be parsed:

The parser raises InvalidFileException when the file cannot be parsed.

For the XML format, _PlistParser.parse() calls expat's ParserCreate().ParseFile() with no exception translation at all. Two classes of malformed input escape as the underlying exception instead, and neither is a ValueError (InvalidFileException's base class), so code that follows the documented contract and catches InvalidFileException (or even just ValueError) does not catch these:

1. XML that is not well-formed raises a raw xml.parsers.expat.ExpatError.
2. An <?xml ... ?> declaration naming an encoding Python's codec registry doesn't know raises a raw LookupError. This is what CIFuzz found in gh-152211 (LookupError: unknown encoding: Latin-5_________________________); the well-formedness case above is a second, broader instance of the same missing-translation bug.

>>> import plistlib
>>> plistlib.loads(b"<plist><foo></bar></plist>")
Traceback (most recent call last):
  ...
xml.parsers.expat.ExpatError: mismatched tag: line 1, column 14
>>> plistlib.loads(b'<?xml version="1.0" encoding="BogusEncoding"?><plist></plist>')
Traceback (most recent call last):
  ...
LookupError: unknown encoding: BogusEncoding

Expected in both cases: plistlib.InvalidFileException, as already happens for other malformed inputs (e.g. plistlib.loads(b"not a plist") correctly raises InvalidFileException).

Related: gh-152211 (the CIFuzz report for the LookupError case specifically; discussion there is about the fuzz-target/CI setup, not a fix to plistlib itself).

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155398
• gh-156177

Start at plistlib._PlistParser.parse, which is called by plistlib.load() and loads(), and reproduce both malformed XML examples from the issue. Confirm that malformed XML and unknown encodings are translated to plistlib.InvalidFileException, then verify the existing malformed-input behavior remains intact.