gh-140950: Read pushed shlex source after EOF

[ThanhNguyxn]

Summary

• Reset lexer state in shlex.push_source() so pushing a new source after EOF can continue tokenization.
• Add regression test test_push_source_after_eof in Lib/test/test_shlex.py covering both POSIX and non-POSIX modes.
• Add NEWS entry for gh-140950.

Repro

from shlex import shlex
lexer = shlex('a')
lexer.get_token()  # 'a'
lexer.get_token()  # EOF
lexer.push_source('b')
lexer.get_token()  # now returns 'b'

Verification

• python -m py_compile Lib/shlex.py Lib/test/test_shlex.py
• python -c "import importlib.util, sys, unittest; shlex_spec=importlib.util.spec_from_file_location('shlex', r'D:\Code\cpython\Lib\shlex.py'); shlex_mod=importlib.util.module_from_spec(shlex_spec); shlex_spec.loader.exec_module(shlex_mod); sys.modules['shlex']=shlex_mod; test_spec=importlib.util.spec_from_file_location('test_shlex_local', r'D:\Code\cpython\Lib\test\test_shlex.py'); test_mod=importlib.util.module_from_spec(test_spec); test_spec.loader.exec_module(test_mod); suite=unittest.TestSuite([test_mod.ShlexTest('test_push_source_after_eof')]); result=unittest.TextTestRunner(verbosity=2).run(suite); raise SystemExit(0 if result.wasSuccessful() else 1)"