Move stdlib/3/curses to stdlib/2and3/curses. (#3025)

Also add the A_ITALIC constant, which is new in 3.7.

stdlib/2and3/_curses.pyi

@@ -1,3 +1,4 @@
+import sys
 from typing import Any, BinaryIO, IO, Optional, Tuple, Union, overload
 
 _chtype = Union[str, bytes, int]
@@ -12,6 +13,8 @@ A_COLOR: int
 A_DIM: int
 A_HORIZONTAL: int
 A_INVIS: int
+if sys.version_info >= (3, 7):
+    A_ITALIC: int
 A_LEFT: int
 A_LOW: int
 A_NORMAL: int
@@ -283,17 +286,20 @@ def tigetstr(capname: str) -> bytes: ...
 def tparm(fmt: bytes, i1: int = ..., i2: int = ..., i3: int = ..., i4: int = ..., i5: int = ..., i6: int = ..., i7: int = ..., i8: int = ..., i9: int = ...) -> bytes: ...
 def typeahead(fd: int) -> None: ...
 def unctrl(ch: _chtype) -> bytes: ...
-def unget_wch(ch: _chtype) -> None: ...
+if sys.version_info >= (3, 3):
+    def unget_wch(ch: _chtype) -> None: ...
 def ungetch(ch: _chtype) -> None: ...
 def ungetmouse(id: int, x: int, y: int, z: int, bstate: int) -> None: ...
-def update_lines_cols() -> int: ...
+if sys.version_info >= (3, 5):
+    def update_lines_cols() -> int: ...
 def use_default_colors() -> None: ...
 def use_env(flag: bool) -> None: ...
 
 class error(Exception): ...
 
 class _CursesWindow:
-    encoding: str
+    if sys.version_info >= (3, 3):
+        encoding: str
     @overload
     def addch(self, ch: _chtype, attr: int = ...) -> None: ...
     @overload
@@ -347,10 +353,11 @@ class _CursesWindow:
     def getch(self) -> _chtype: ...
     @overload
     def getch(self, y: int, x: int) -> _chtype: ...
-    @overload
-    def get_wch(self) -> _chtype: ...
-    @overload
-    def get_wch(self, y: int, x: int) -> _chtype: ...
+    if sys.version_info >= (3, 3):
+        @overload
+        def get_wch(self) -> _chtype: ...
+        @overload
+        def get_wch(self, y: int, x: int) -> _chtype: ...
     @overload
     def getkey(self) -> str: ...
     @overload