Initial changes to prepare for SIG(0) (#1202)

Initial changes to prepare for SIG(0) (#1202)

* # This is a combination of 6 commits.
# This is the 1st commit message:

Initial changes to prepare for SIG(0):
- Add shared RRSIGBase for code shared between RRSIG and SIG
- Add KEY RR
- Add SIG RR

# This is the commit message #2:

Parse flags mnemonics and symbolic protocol names

# This is the commit message #3:

RFC 2535 section 7.1 says "Note that if the type flags field has the NOKEY value, nothing appears after the algorithm octet."

# This is the commit message #4:

Include sphinx only for Python 3.11 or later (#1225)

* Include sphinx only for Python 3.11 or later

* Use python_version
# This is the commit message #5:

Save token before returning it (for exception handling)

# This is the commit message #6:

Replace get/unget with plain unget and last token

* Initial changes to prepare for SIG(0):
- Add shared RRSIGBase for code shared between RRSIG and SIG
- Add KEY RR
- Add SIG RR
- Parse flags mnemonics and symbolic protocol names

* Make pyright happy