gh-144702: Use standard terminology in class pattern error message

[sharkdp]

When trying to use a class pattern to match on a non-class object, we currently get the following error message:

Traceback (most recent call last):
  File "/tmp/test.py", line 2, in <module>
    case len():
         ~~~^^
TypeError: called match pattern must be a class

The terminology "called match pattern" here is a bit confusing. It's not used anywhere else as far as I can tell, and for a beginner, it might read like something is actually being called here.

The documentation consistently refers to these patterns as "class patterns":

• https://docs.python.org/3/reference/compound_stmts.html#class-patterns
• https://peps.python.org/pep-0635/#class-patterns
• https://docs.python.org/3/library/ast.html#ast.MatchClass
• https://docs.python.org/3/reference/datamodel.html#customizing-positional-arguments-in-class-pattern-matching

Here, we change this message to "class pattern must refer to a class".

• Issue: Use standard terminology in class pattern error message #144702

[johnslavik]

I assume we don't need a test. Without the entire check, _PyEval_MatchClass will still appropriately fail with a TypeError.