gh-142624: Make test_inspect's _pickle requirement optional

[youknowone]

• Issue: _pickle is mandatory for test_inspect #142624

[aisk]

Suggested change

	try:
	import _pickle
	MISSING_C_PICKLE = False
	except ImportError:
	MISSING_C_PICKLE = True
	try:
	import _pickle
	except ImportError:
	_pickle = None

This pattern is used more often in tests, as shown in the line below, it removes the need for a variable and makes the codebase more consistent.

[zware]

With this, use @unittest.skipUnless(_pickle, "requires _pickle module") as the test decorator.

Or to pick a different pattern in use in this module, use _pickle = import_helper.import_module('_pickle') in the test (see some examples that use _testcapi).

As another option, is there a particular reason to use _pickle? Can/should something from _testcapi be used instead?