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.

WORKSPACE

@@ -3,3 +3,42 @@ workspace(name = "com_github_grpc_grpc")
 load("//bazel:grpc_deps.bzl", "grpc_deps", "grpc_test_only_deps")
 grpc_deps()
 grpc_test_only_deps()
+
+new_http_archive(
+    name="cython",
+    sha256="d68138a2381afbdd0876c3cb2a22389043fa01c4badede1228ee073032b07a27",
+    urls=[
+        "https://github.com/cython/cython/archive/c2b80d87658a8525ce091cbe146cb7eaa29fed5c.tar.gz",
+    ],
+    strip_prefix="cython-c2b80d87658a8525ce091cbe146cb7eaa29fed5c",
+    build_file="//third_party:cython.BUILD",
+)
+
+load("//third_party/py:python_configure.bzl", "python_configure")
+python_configure(name="local_config_python")
+
+git_repository(
+    name="io_bazel_rules_python",
+    remote="https://github.com/bazelbuild/rules_python.git",
+    commit="8b5d0683a7d878b28fffe464779c8a53659fc645",
+)
+
+load("@io_bazel_rules_python//python:pip.bzl", "pip_repositories", "pip_import")
+
+pip_repositories()
+pip_import(
+    name="grpc_python_dependencies",
+    requirements="//:requirements.bazel.txt",
+)
+
+load("@grpc_python_dependencies//:requirements.bzl", "pip_install")
+pip_install()
+
+git_repository(
+    name="org_pubref_rules_protobuf",
+    remote="https://github.com/pubref/rules_protobuf",
+    tag="v0.8.2",
+)
+
+load("@org_pubref_rules_protobuf//python:rules.bzl", "py_proto_repositories")
+py_proto_repositories()

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

third_party/BUILD

@@ -8,4 +8,5 @@ exports_files([
     "incremental.BUILD",
     "zope_interface.BUILD",
     "constantly.BUILD",
+    "cython.BUILD",
 ])