Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/warnings.pyi

@@ -0,0 +1,73 @@
+import sys
+from types import ModuleType, TracebackType
+from typing import Any, List, Optional, TextIO, Type, Union, overload
+from typing_extensions import Literal
+
+from _warnings import warn as warn, warn_explicit as warn_explicit
+
+def showwarning(
+    message: Union[Warning, str],
+    category: Type[Warning],
+    filename: str,
+    lineno: int,
+    file: Optional[TextIO] = ...,
+    line: Optional[str] = ...,
+) -> None: ...
+def formatwarning(
+    message: Union[Warning, str], category: Type[Warning], filename: str, lineno: int, line: Optional[str] = ...
+) -> str: ...
+def filterwarnings(
+    action: str, message: str = ..., category: Type[Warning] = ..., module: str = ..., lineno: int = ..., append: bool = ...
+) -> None: ...
+def simplefilter(action: str, category: Type[Warning] = ..., lineno: int = ..., append: bool = ...) -> None: ...
+def resetwarnings() -> None: ...
+
+class _OptionError(Exception): ...
+
+class WarningMessage:
+    message: Union[Warning, str]
+    category: Type[Warning]
+    filename: str
+    lineno: int
+    file: Optional[TextIO]
+    line: Optional[str]
+    if sys.version_info >= (3, 6):
+        source: Optional[Any]
+        def __init__(
+            self,
+            message: Union[Warning, str],
+            category: Type[Warning],
+            filename: str,
+            lineno: int,
+            file: Optional[TextIO] = ...,
+            line: Optional[str] = ...,
+            source: Optional[Any] = ...,
+        ) -> None: ...
+    else:
+        def __init__(
+            self,
+            message: Union[Warning, str],
+            category: Type[Warning],
+            filename: str,
+            lineno: int,
+            file: Optional[TextIO] = ...,
+            line: Optional[str] = ...,
+        ) -> None: ...
+
+class catch_warnings:
+    @overload
+    def __new__(cls, *, record: Literal[False] = ..., module: Optional[ModuleType] = ...) -> _catch_warnings_without_records: ...
+    @overload
+    def __new__(cls, *, record: Literal[True], module: Optional[ModuleType] = ...) -> _catch_warnings_with_records: ...
+    @overload
+    def __new__(cls, *, record: bool, module: Optional[ModuleType] = ...) -> catch_warnings: ...
+    def __enter__(self) -> Optional[List[WarningMessage]]: ...
+    def __exit__(
+        self, exc_type: Optional[Type[BaseException]], exc_val: Optional[BaseException], exc_tb: Optional[TracebackType]
+    ) -> None: ...
+
+class _catch_warnings_without_records(catch_warnings):
+    def __enter__(self) -> None: ...
+
+class _catch_warnings_with_records(catch_warnings):
+    def __enter__(self) -> List[WarningMessage]: ...