bpo-47040: improve document of checksum functions (gh-31955)

Clarifies a versionchanged note on crc32 & adler32 docs that the workaround is only needed for Python 2 and earlier.
Also cleans up an unnecessary intermediate variable in the implementation.

Authored-By: Ma Lin / animalize
Co-authored-by: Gregory P. Smith <greg@krypto.org>