Add LICENSE files to sdists (#4248)

Fixes #4247

MANIFEST.in

@@ -4,3 +4,4 @@ recursive-include extensions *
 recursive-include docs *
 include runtests.py
 include mypy_self_check.ini
+include LICENSE

extensions/LICENSE

@@ -0,0 +1,27 @@
+Mypy extensions are licensed under the terms of the MIT license, reproduced below.
+
+= = = = =
+
+The MIT License
+
+Copyright (c) 2016-2017 Jukka Lehtosalo and contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+
+= = = = =

extensions/MANIFEST.in

@@ -0,0 +1 @@
+include LICENSE