pybind: hardwire language_level to 3

pybind: hardwire language_level to 3

Previously, to maintain backward compatibility with Python 2, we set
'language_level' to sys.version_info.major, so the value would be 2
when building with Python 2, and 3 with Python 3. Now that Python 2
support has been dropped, we can hardwire it to "3".

This change also removes the comment about switching to
`language_level=3str` in the future. According to the Cython 3.1+
documentation,

> language_level=3 is now the default. language_level=3str has become a
> legacy alias.

see https://cython.readthedocs.io/en/3.1.x/src/changes.html.

For context, in Cython < 3.1, language_level=3 and language_level=3str
had different meanings:
- 3 = unprefixed strings are unicode
- 3str = unprefixed strings follow Python version (bytes in Py2, unicode
  in Py3)

Since we no longer support Python 2, this distinction is irrelevant and
the comment can be safely removed.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>