Drop support for Python 3.8

[kkirsche]

With the release of Python 3.13, Python 3.8 is now end of life, as shown on https://devguide.python.org/versions/

By dropping Python 3.8, this expands the available features for unconditional use in the library to include (but is not limited to):

• Dictionary merge and update operators
• Type Hinting Generics in Standard Collections
• New string methods to remove prefixes and suffixes
• hashlib now can use SHA3 and SHAKE XOF from OpenSSL
• typing.Annotated is now supported

If more complete typing information is desired, additional work can be done to incorporate typing-extensions so that things like kwargs can be strongly typed via Unpack