gh-129666: Add C11/C++11 to docs and -pedantic-errors to GCC/clang test_c[pp]ext tests (GH-130692)

Disable pedantic check for c++03 (unlimited API)

Also add a check for c++03 *limited* API, which passes in pedantic mode
after removing a comma in the `PySendResult` declaration, and allowing
`long long`.

Lib/test/test_cppext/setup.py

@@ -19,6 +19,17 @@
         # warnings
         '-Werror',
     ]
+
+    CPPFLAGS_PEDANTIC = [
+        # Ask for strict(er) compliance with the standard.
+        # We cannot do this for c++03 unlimited API, since several headers in
+        # Include/cpython/ use commas at end of `enum` declarations, a C++11
+        # feature for which GCC has no narrower option than -Wpedantic itself.
+        '-pedantic-errors',
+
+        # We also use `long long`, a C++11 feature we can enable individually.
+        '-Wno-long-long',
+    ]
 else:
     # MSVC compiler flags
     CPPFLAGS = [
@@ -27,6 +38,7 @@
         # Treat all compiler warnings as compiler errors
         '/WX',
     ]
+    CPPFLAGS_PEDANTIC = []
 
 
 def main():
@@ -45,6 +57,10 @@ def main():
         else:
             cppflags.append(f'-std={std}')
 
+        if limited or (std != 'c++03'):
+            # See CPPFLAGS_PEDANTIC docstring
+            cppflags.extend(CPPFLAGS_PEDANTIC)
+
     # gh-105776: When "gcc -std=11" is used as the C++ compiler, -std=c11
     # option emits a C++ compiler warning. Remove "-std11" option from the
     # CC command.