implement some minor 3.7 features (#1966)

References:
- https://docs.python.org/dev/library/datetime.html#datetime.datetime.fromisoformat
- https://docs.python.org/dev/library/crypt.html#crypt.mksalt
- https://docs.python.org/dev/library/contextlib.html#contextlib.AbstractAsyncContextManager
- https://docs.python.org/dev/library/calendar.html#calendar.HTMLCalendar.cssclasses
- https://docs.python.org/dev/library/binascii.html#binascii.b2a_uu

Part of #1965.

stdlib/2and3/binascii.pyi

@@ -21,7 +21,10 @@ else:
     _Ascii = Union[bytes, Text]
 
 def a2b_uu(string: _Ascii) -> bytes: ...
-def b2a_uu(data: _Bytes) -> bytes: ...
+if sys.version_info >= (3, 7):
+    def b2a_uu(data: _Bytes, *, backtick: bool = ...) -> bytes: ...
+else:
+    def b2a_uu(data: _Bytes) -> bytes: ...
 def a2b_base64(string: _Ascii) -> bytes: ...
 if sys.version_info >= (3, 6):
     def b2a_base64(data: _Bytes, *, newline: bool = ...) -> bytes: ...

stdlib/2and3/calendar.pyi

@@ -65,6 +65,14 @@ class HTMLCalendar(Calendar):
     def formatmonth(self, theyear: int, themonth: int, withyear: bool = ...) -> str: ...
     def formatyear(self, theyear: int, width: int = ...) -> str: ...
     def formatyearpage(self, theyear: int, width: int = ..., css: Optional[str] = ..., encoding: Optional[str] = ...) -> str: ...
+    if sys.version_info >= (3, 7):
+        cssclasses: List[str]
+        cssclass_today: str
+        cssclasses_weekday_head: List[str]
+        cssclass_month_head: str
+        cssclass_month: str
+        cssclass_year: str
+        cssclass_year_head: str
 
 if sys.version_info < (3, 0):
     class TimeEncoding:

stdlib/2and3/contextlib.pyi

@@ -14,6 +14,8 @@ if sys.version_info >= (3, 5):
 
 if sys.version_info >= (3, 6):
     from typing import ContextManager as AbstractContextManager
+if sys.version_info >= (3, 7):
+    from typing import AsyncContextManager as AbstractAsyncContextManager
 
 _T = TypeVar('_T')
 

stdlib/2and3/crypt.pyi

@@ -9,10 +9,15 @@ if sys.version_info >= (3, 3):
     METHOD_MD5: _Method
     METHOD_SHA256: _Method
     METHOD_SHA512: _Method
+    if sys.version_info >= (3, 7):
+        METHOD_BLOWFISH: _Method
 
     methods: List[_Method]
 
-    def mksalt(method: Optional[_Method] = ...) -> str: ...
+    if sys.version_info >= (3, 7):
+        def mksalt(method: Optional[_Method] = ..., *, rounds: Optional[int] = ...) -> str: ...
+    else:
+        def mksalt(method: Optional[_Method] = ...) -> str: ...
     def crypt(word: str, salt: Optional[Union[str, _Method]] = ...) -> str: ...
 else:
     def crypt(word: str, salt: str) -> str: ...

stdlib/2and3/datetime.pyi

@@ -223,6 +223,9 @@ class datetime:
     else:
         @classmethod
         def combine(cls, date: date, time: time) -> datetime: ...
+    if sys.version_info >= (3, 7):
+        @classmethod
+        def fromisoformat(cls, date_string: str) -> datetime: ...
     def strftime(self, fmt: _Text) -> str: ...
     if sys.version_info >= (3,):
         def __format__(self, fmt: str) -> str: ...