Update stale Python 2 comments and docstrings

Several comments and docstrings still referenced Python 2 concepts that
no longer apply now that the driver requires Python 3.9+:

- encoder.py: Updated cql_encode_unicode() docstring to note it is
  unused since Python 2 removal (str is always unicode on Python 3).
  Also fixed the method body: it was calling val.encode('utf-8') which
  on Python 3 converts str to bytes, producing wrong output. Now it
  passes val directly to cql_quote.

- metadata.py: Changed 'will always be a unicode' to 'will always be
  a str' (line 2155). Updated unhexlify comment to say 'str input'
  instead of 'unicode input' and fixed typo 'everythin' (line 2350).

- graphson.py: Removed '(PY2)'/'(PY3)' qualifiers from the type
  mapping table. Updated 'long' to 'int' for varint, 'str (unicode)'
  to 'str' for inet, removed 'buffer (PY2)' from blob entries.

- util.py: Updated comment on _positional_rename_invalid_identifiers
  to remove stale 'Python 2.6' reference.

- asyncorereactor.py: Removed stale 'TODO: Remove when Python 2
  support is removed' since Python 2 support has been removed. The
  guard itself is still needed for interpreter shutdown scenarios.

All 608 unit tests pass.

Author: mykaul