Workspace changes to build gRPC Python with Bazel

- Include new rules from bazelbuild/rules_python, which allow for
importing pip dependencies with Bazel.
- Make Cython BUILD file visible to WORKSPACE.
- Use a custom requirements file for Bazel python. This is added to
allow for requirements previously added only during build steps.
diff --git a/requirements.bazel.txt b/requirements.bazel.txt
new file mode 100644
index 0000000..16f31f9
--- /dev/null
+++ b/requirements.bazel.txt
@@ -0,0 +1,10 @@
+# GRPC Python setup requirements
+coverage>=4.0
+cython==0.28.3
+enum34>=1.0.4
+protobuf>=3.5.0.post1
+six>=1.10
+wheel>=0.29
+futures>=2.2.0
+google-auth>=1.0.0
+oauth2client==4.1.0