add codecs.StreamRecoder attributes

[tungol]

That these were missing from the stubs was being silenced by this part of the common allowlist:

# See comments in file. List out methods that are delegated by __getattr__ at runtime.
# Used to make the relevant class satisfy BinaryIO interface.
codecs.StreamReaderWriter.\w+
codecs.StreamRecoder.\w+

But these weren't part of the group of errors being described, which are errors about things being in the stubs that don't appear to exist at runtime.

[github-actions]

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