gh-150544: Fix pdb _pyrepl_available() and make test_gen_colors skip at runtime

[sendaoYan]

Summary

Fix pdb._pyrepl_available() so it checks the actual stdin/stdout streams (including TTY and fileno() support), not only the import-time CAN_USE_PYREPL flag.
Pass self.stdin and self.stdout from Pdb.init into _pyrepl_available().
Update test_gen_colors and PdbTestReadlinePyREPL to evaluate pyrepl availability at test runtime instead of via import-time @unittest.skipIf.
Harden test_pyrepl_available with explicit mock streams.
Fixes gh-150544

Problem

test.test_pdb.PdbTestColorize.test_gen_colors could fail with:

Traceback (most recent call last):
  File �[35m"/home/yansendao/git/cpython-ysd/Lib/test/test_pdb.py"�[0m, line �[35m5008�[0m, in �[35mtest_gen_colors�[0m
    gen_colors = �[1;31mp.pyrepl_input.gen_colors�[0m
                 �[1;31m^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
�[1;35mAttributeError�[0m: �[35m'NoneType' object has no attribute 'gen_colors'�[0m

Changes

• Lib/pdb.py

Extend _pyrepl_available(stdin=None, stdout=None) to:
honor PYTHON_BASIC_REPL and CAN_USE_PYREPL
verify stdin is a TTY at call time
verify both streams support fileno()
handle fake stdin objects (e.g. doctest FakeInput) without raising
Call _pyrepl_available(self.stdin, self.stdout) from Pdb.init

• Lib/test/test_pdb.py

test_gen_colors: replace import-time @skipIf with runtime skipTest when pyrepl or pyrepl_input is unavailable
PdbTestReadlinePyREPL: use setUpClass for runtime skipping
test_pyrepl_available: test TTY and non-TTY cases using mock streams

Testing

All the cpython tests shows this PR do not cause new fails.