[Pixeless001]

Fills in the remaining Incomplete annotations outside the deprecated ssl dict, based on PyMySQL 1.2.3:

• read_length_encoded_integer returns an int, or None for a NULL column
• sha1_new is functools.partial(hashlib.new, "sha1")
• caching_sha2_password_auth returns the last packet, or None when it falls through after the RSA exchange
• MySQLResult.description and rows match what _get_descriptions() and _read_rowdata_packet() build

Also drops MySQLResult.first_packet, which is a local variable in read() and not an attribute.

[github-actions]

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