Minor fixes and updates in trait_types.pyi; new ctraits.pyi (#1661)

This PR makes some minor type-stub related fixes and cleanups, as part of an incremental effort to get the stubs into a usable state.

* Add suitable flake8 exclusions for type stubs (it's common to put the ... on the same line as a function def, for example; black does that, but flake8 complains about it).
* Add new ctraits.pyi type stubs. It's not complete, but it's sufficient to stop complaints in some other parts of the codebase.
* Fix confusion with int being both the built-in type and an enum name in ValidateTrait
* Several fixes in trait_types.pyi. In particular, Pylance/Pyright doesn't like missing type variables, even though the typing documentation says that it should be okay to use e.g., plain Callable instead of Callable[..., Any].
* Fix some non-standard double indentation in the trait_types.pyi stubs.

Co-authored-by: Corran Webster <cwebster@enthought.com>
(cherry picked from commit 9f5fe4d)

setup.cfg

@@ -1,4 +1,7 @@
 [flake8]
 exclude = traits/observation/_generated_parser.py,build
 ignore = E266,W503,E722,E731,E741
-per-file-ignores = */api.py:F401
+per-file-ignores =
+    */api.py:F401
+    # Suppress flake8 complaints about black's formatting of .pyi files
+    *.pyi:E302,E305,E701,E704

traits-stubs/traits-stubs/constants.pyi

@@ -11,6 +11,7 @@
 from enum import IntEnum
 from typing import Any
 
+
 class TraitKind(IntEnum):
     trait: int = ...
     python: int = ...
@@ -22,26 +23,31 @@ class TraitKind(IntEnum):
     constant: int = ...
     generic: int = ...
 
+
+# Alias for the built-in int type, to avoid confusion with the 'int' attribute
+# in the ValidateTrait namespace.
+_int = int
+
 class ValidateTrait(IntEnum):
-    type: int = ...
-    instance: int = ...
-    self_type: int = ...
-    int_range: int = ...
-    float_range: int = ...
-    enum: int = ...
-    map: int = ...
-    complex: int = ...
-    slow: int = ...
-    tuple: int = ...
-    prefix_map: int = ...
-    coerce: int = ...
-    cast: int = ...
-    function: int = ...
-    python: int = ...
-    adapt: int = ...
-    int: int = ...
-    float: int = ...
-    callable: int = ...
+    type: _int = ...
+    instance: _int = ...
+    self_type: _int = ...
+    int_range: _int = ...
+    float_range: _int = ...
+    enum: _int = ...
+    map: _int = ...
+    complex: _int = ...
+    slow: _int = ...
+    tuple: _int = ...
+    prefix_map: _int = ...
+    coerce: _int = ...
+    cast: _int = ...
+    function: _int = ...
+    python: _int = ...
+    adapt: _int = ...
+    int: _int = ...
+    float: _int = ...
+    callable: _int = ...
 
 class ComparisonMode(IntEnum):
     none: int = ...

traits-stubs/traits-stubs/ctraits.pyi

@@ -0,0 +1,28 @@
+# (C) Copyright 2020-2022 Enthought, Inc., Austin, TX
+# All rights reserved.
+#
+# This software is provided without warranty under the terms of the BSD
+# license included in LICENSE.txt and may be redistributed only under
+# the conditions described in the aforementioned license. The license
+# is also available online at http://www.enthought.com/licenses/BSD.txt
+#
+# Thanks for using Enthought open source!
+
+from typing import Any
+
+# Constants used in DefaultValue enumeration.
+_CALLABLE_AND_ARGS_DEFAULT_VALUE: int
+_CALLABLE_DEFAULT_VALUE: int
+_CONSTANT_DEFAULT_VALUE: int
+_DICT_COPY_DEFAULT_VALUE: int
+_DISALLOW_DEFAULT_VALUE: int
+_LIST_COPY_DEFAULT_VALUE: int
+_MAXIMUM_DEFAULT_VALUE_TYPE: int
+_MISSING_DEFAULT_VALUE: int
+_OBJECT_DEFAULT_VALUE: int
+_TRAIT_DICT_OBJECT_DEFAULT_VALUE: int
+_TRAIT_LIST_OBJECT_DEFAULT_VALUE: int
+_TRAIT_SET_OBJECT_DEFAULT_VALUE: int
+
+def _validate_complex_number(value: Any) -> complex: ...
+def _validate_float(value: Any) -> float: ...