Add TLSVersion & related attributes to SSLContext (#3097)

Author: hynek

stdlib/2and3/ssl.pyi

@@ -211,6 +211,17 @@ class SSLSocket(socket.socket):
     def pending(self) -> int: ...
 
 
+if sys.version_info >= (3, 7):
+    class TLSVersion(enum.IntEnum):
+        MINIMUM_SUPPORTED = ...
+        MAXIMUM_SUPPORTED = ...
+        SSLv3 = ...
+        TLSv1 = ...
+        TLSv1_1 = ...
+        TLSv1_2 = ...
+        TLSv1_3 = ...
+
+
 class SSLContext:
     if sys.version_info < (3,) or sys.version_info >= (3, 4):
         check_hostname: bool
@@ -257,6 +268,9 @@ class SSLContext:
                      server_side: bool = ...,
                      server_hostname: Optional[str] = ...) -> SSLObject: ...
     def session_stats(self) -> Dict[str, int]: ...
+    if sys.version_info >= (3, 7):
+        maximum_version: TLSVersion
+        minimum_version: TLSVersion
 
 
 if sys.version_info >= (3, 5):