Set literal default for bz2.BZ2Compressor.__init__

[brianschubert]

Another case missed by stubdefaulter.

For reference:

• https://docs.python.org/3/library/bz2.html#bz2.BZ2Compressor

>>> import bz2, inspect
>>> inspect.signature(bz2.BZ2Compressor)
<Signature (compresslevel=9, /)>

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉