Add empty folly/python/futures.pyx

Reviewed By: yfeldblum

Differential Revision: D16214195

fbshipit-source-id: 4841b0ba3188f19aba7a0e2c2ffd04d2f5f55220

folly/python/futures.pyx

+# Copyright 2019-present Facebook, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.