gh-108494: Argument clinic: Improve the parse_file() API

[AlexWaygood]

• Get rid of the ns argument to parse_file(); restore the verify argument that was removed in 1dd9510.
• Add a limited_capi argument to parse_file(). Make the limited_capi argument to both Clinic.__init__ and parse_file required, so that callers always have to be explicit about whether the limited C API is desired or not. This means that there is a "single source of truth" about what the default is, rather than this being duplicated between the global constant and the logic in the create_cli() function.
• Get rid of MockClinic from the test file, which was added in 1dd9510: just use our existing _make_clinic() helper function instead.

With this PR, we would need to make only these changes if we wanted to change it tomorrow so that the limited C API is the default for AC-generated code (plus a few fixes to some tests that currently depend on being able to use our internal C API):

diff --git a/Lib/test/test_clinic.py b/Lib/test/test_clinic.py
index efc6a922c5..f4f8b2dd20 100644
--- a/Lib/test/test_clinic.py
+++ b/Lib/test/test_clinic.py
@@ -20,7 +20,7 @@
     import clinic
     from clinic import DSLParser

-DEFAULT_LIMITED_C_API = False
+DEFAULT_LIMITED_C_API = True


 def _make_clinic(*, filename='clinic_tests'):
diff --git a/Tools/clinic/clinic.py b/Tools/clinic/clinic.py
index 2a4b6ff3b7..1a82ff7e57 100755
--- a/Tools/clinic/clinic.py
+++ b/Tools/clinic/clinic.py
@@ -6096,8 +6096,8 @@ def create_cli() -> argparse.ArgumentParser:
     cmdline.add_argument("--exclude", type=str, action="append",
                          help=("a file to exclude in --make mode; "
                                "can be given multiple times"))
-    cmdline.add_argument("--limited", dest="limited_capi", action='store_true',
-                         help="use the Limited C API")
+    cmdline.add_argument("--no-limited", dest="limited_capi", action='store_false',
+                         help="Don't use the Limited C API")
     cmdline.add_argument("filename", metavar="FILE", type=str, nargs="*",
                          help="the list of files to process")
     return cmdline

Closes #108504

• Issue: Add support for the limited C API to Argument Clinic #108494

[vstinner]

LGTM, whatever works :-) I'm not going to nitpick on the coding style ;-)

[AlexWaygood]

Thanks for the reviews!