pythongh-108455: Simplify the peg_generator mypy config (python#108620)

make it easier to see exactly which options from mypy's `--strict` mode can't currently be enabled

Tools/peg_generator/mypy.ini

@@ -1,27 +1,17 @@
 [mypy]
 files = Tools/peg_generator/pegen
 pretty = True
+show_traceback = True
 
-follow_imports = error
-no_implicit_optional = True
-strict_optional = True
-
-#check_untyped_defs = True
-disallow_untyped_calls = True
-disallow_untyped_defs = True
-
-disallow_any_generics = true
-disallow_any_unimported = True
-disallow_incomplete_defs = True
-disallow_subclassing_any = True
+# Make sure the peg_generator can be run using Python 3.10:
+python_version = 3.10
 
-warn_unused_configs = True
-warn_unused_ignores = true
-warn_redundant_casts = true
-warn_no_return = True
+# Be strict...
+strict = True
 
-show_traceback = True
-show_error_codes = True
+# except for a few settings that can't yet be enabled:
+warn_return_any = False
+no_implicit_reexport = False
 
 [mypy-pegen.grammar_parser]
 strict_optional = False