support unicode in Python 2 for difflib (#2055)

Fixes #1961.

I mostly just replaced all str annotations with Text, including in return types. This is
only broadly correct; diffing a str and a unicode sequence actually results in a mixed
output of str and unicode. We could also keep the return types as str if using Text
causes errors in real code. For callbacks that take str, I introduced a Union alias
because a callable taking a str would not be a compatible with a parameter of type
Callable[[Text], bool].

I also fixed the return type of difflib.restore.

stdlib/2and3/difflib.pyi

@@ -1,14 +1,13 @@
 # Based on https://docs.python.org/2.7/library/difflib.html and https://docs.python.org/3.2/library/difflib.html
 
-# TODO: Support unicode in Python 2?
-
 import sys
 from typing import (
     TypeVar, Callable, Iterable, Iterator, List, NamedTuple, Sequence, Tuple,
-    Generic, Optional
+    Generic, Optional, Text, Union
 )
 
 _T = TypeVar('_T')
+_JunkCallback = Union[Callable[[Text], bool], Callable[[str], bool]]
 
 Match = NamedTuple('Match', [
     ('a', int),
@@ -37,36 +36,35 @@ def get_close_matches(word: Sequence[_T], possibilities: Iterable[Sequence[_T]],
                       n: int = ..., cutoff: float = ...) -> List[Sequence[_T]]: ...
 
 class Differ:
-    def __init__(self, linejunk: Callable[[str], bool] = ...,
-                 charjunk: Callable[[str], bool] = ...) -> None: ...
-    def compare(self, a: Sequence[str], b: Sequence[str]) -> Iterator[str]: ...
+    def __init__(self, linejunk: _JunkCallback = ..., charjunk: _JunkCallback = ...) -> None: ...
+    def compare(self, a: Sequence[Text], b: Sequence[Text]) -> Iterator[Text]: ...
 
-def IS_LINE_JUNK(str) -> bool: ...
-def IS_CHARACTER_JUNK(str) -> bool: ...
-def unified_diff(a: Sequence[str], b: Sequence[str], fromfile: str = ...,
-                 tofile: str = ..., fromfiledate: str = ..., tofiledate: str = ...,
-                 n: int = ..., lineterm: str = ...) -> Iterator[str]: ...
-def context_diff(a: Sequence[str], b: Sequence[str], fromfile: str=...,
-                 tofile: str = ..., fromfiledate: str = ..., tofiledate: str = ...,
-                 n: int = ..., lineterm: str = ...) -> Iterator[str]: ...
-def ndiff(a: Sequence[str], b: Sequence[str],
-          linejunk: Callable[[str], bool] = ...,
-          charjunk: Callable[[str], bool] = ...
-          ) -> Iterator[str]: ...
+def IS_LINE_JUNK(line: Text) -> bool: ...
+def IS_CHARACTER_JUNK(line: Text) -> bool: ...
+def unified_diff(a: Sequence[Text], b: Sequence[Text], fromfile: Text = ...,
+                 tofile: Text = ..., fromfiledate: Text = ..., tofiledate: Text = ...,
+                 n: int = ..., lineterm: Text = ...) -> Iterator[Text]: ...
+def context_diff(a: Sequence[Text], b: Sequence[Text], fromfile: Text=...,
+                 tofile: Text = ..., fromfiledate: Text = ..., tofiledate: Text = ...,
+                 n: int = ..., lineterm: Text = ...) -> Iterator[Text]: ...
+def ndiff(a: Sequence[Text], b: Sequence[Text],
+          linejunk: _JunkCallback = ...,
+          charjunk: _JunkCallback = ...
+          ) -> Iterator[Text]: ...
 
 class HtmlDiff(object):
     def __init__(self, tabsize: int = ..., wrapcolumn: int = ...,
-                 linejunk: Callable[[str], bool] = ...,
-                 charjunk: Callable[[str], bool] = ...
+                 linejunk: _JunkCallback = ...,
+                 charjunk: _JunkCallback = ...
                  ) -> None: ...
-    def make_file(self, fromlines: Sequence[str], tolines: Sequence[str],
-                  fromdesc: str = ..., todesc: str = ..., context: bool = ...,
-                  numlines: int = ...) -> str: ...
-    def make_table(self, fromlines: Sequence[str], tolines: Sequence[str],
-                   fromdesc: str = ..., todesc: str = ..., context: bool = ...,
-                   numlines: int = ...) -> str: ...
+    def make_file(self, fromlines: Sequence[Text], tolines: Sequence[Text],
+                  fromdesc: Text = ..., todesc: Text = ..., context: bool = ...,
+                  numlines: int = ...) -> Text: ...
+    def make_table(self, fromlines: Sequence[Text], tolines: Sequence[Text],
+                   fromdesc: Text = ..., todesc: Text = ..., context: bool = ...,
+                   numlines: int = ...) -> Text: ...
 
-def restore(delta: Iterable[str], which: int) -> Iterator[int]: ...
+def restore(delta: Iterable[Text], which: int) -> Iterator[Text]: ...
 
 if sys.version_info >= (3, 5):
     def diff_bytes(