feat(mypy): Change configuration format

Changed mypy configuration INI format.  Python's configparser checks
boolean values in a case-insensitive manner, so it doesn't matter if
values start in Title-case or lower-case.  This way, the configuration
is easily translated to TOML format for writing a pyproject.toml file;
with the exception of string values which will have to be enclosed in
quotes anyways.

mypy/config

@@ -1,19 +1,19 @@
 [mypy]
-show_column_numbers = True
-ignore_missing_imports = True
+show_column_numbers = true
+ignore_missing_imports = true
 follow_imports = silent
-show_error_codes = True
+show_error_codes = true
 
-warn_unused_configs = True
-allow_any_generics = False
-allow_subclassing_any = False
-allow_untyped_calls = False
-allow_untyped_defs = False
-allow_incomplete_defs = False
-check_untyped_defs = True
-allow_untyped_decorators = False
-no_implicit_optional = True
-warn_redundant_casts = True
-warn_unused_ignores = True
-warn_return_any = True
-implicit_reexport = False
+warn_unused_configs = true
+allow_any_generics = false
+allow_subclassing_any = false
+allow_untyped_calls = false
+allow_untyped_defs = false
+allow_incomplete_defs = false
+check_untyped_defs = true
+allow_untyped_decorators = false
+no_implicit_optional = true
+warn_redundant_casts = true
+warn_unused_ignores = true
+warn_return_any = true
+implicit_reexport = false