Add stub for filecmp to 2and3 (#955)

* Add stub for filecmp to 2and3

* Fix spacing

* Moar spacing

* Use _SupportsBool

* Code review

stdlib/2and3/filecmp.pyi

@@ -0,0 +1,48 @@
+# Stubs for filecmp (Python 2/3)
+import sys
+from typing import AnyStr, Callable, Dict, Generic, Iterable, List, Optional, Sequence, Tuple, Union, Text
+
+DEFAULT_IGNORES = ...  # type: List[str]
+
+def cmp(f1: Union[bytes, Text], f2: Union[bytes, Text], shallow: Union[int, bool] = ...) -> bool: ...
+def cmpfiles(a: AnyStr, b: AnyStr, common: Iterable[AnyStr],
+             shallow: Union[int, bool] = ...) -> Tuple[List[AnyStr], List[AnyStr], List[AnyStr]]: ...
+
+class dircmp(Generic[AnyStr]):
+    def __init__(self, a: AnyStr, b: AnyStr,
+                 ignore: Optional[Sequence[AnyStr]] = ...,
+                 hide: Optional[Sequence[AnyStr]] = ...) -> None: ...
+
+    left = ...  # type: AnyStr
+    right = ...  # type: AnyStr
+    hide = ...  # type: Sequence[AnyStr]
+    ignore = ...  # type: Sequence[AnyStr]
+
+    # These properties are created at runtime by __getattr__
+    subdirs = ...  # type: Dict[AnyStr, dircmp[AnyStr]]
+    same_files = ...  # type: List[AnyStr]
+    diff_files = ...  # type: List[AnyStr]
+    funny_files = ...  # type: List[AnyStr]
+    common_dirs = ...  # type: List[AnyStr]
+    common_files = ...  # type: List[AnyStr]
+    common_funny = ...  # type: List[AnyStr]
+    common = ...  # type: List[AnyStr]
+    left_only = ...  # type: List[AnyStr]
+    right_only = ...  # type: List[AnyStr]
+    left_list = ...  # type: List[AnyStr]
+    right_list = ...  # type: List[AnyStr]
+
+    def report(self) -> None: ...
+    def report_partial_closure(self) -> None: ...
+    def report_full_closure(self) -> None: ...
+
+    methodmap = ...  # type: Dict[str, Callable[[], None]]
+    def phase0(self) -> None: ...
+    def phase1(self) -> None: ...
+    def phase2(self) -> None: ...
+    def phase3(self) -> None: ...
+    def phase4(self) -> None: ...
+    def phase4_closure(self) -> None: ...
+
+if sys.version_info >= (3,):
+    def clear_cache() -> None: ...