Downgrade rules_python to 0.1.0 to accommodate Kokoro image.

Kokoro still is on Python 3.5. Hopefully soon we will be on
a Bazel Docker image where we can include a newer Python.

bazel/workspace_deps.bzl

@@ -25,8 +25,8 @@ def upb_deps():
     maybe(
         http_archive,
         name = "rules_python",
-        url = "https://github.com/bazelbuild/rules_python/releases/download/0.3.0/rules_python-0.3.0.tar.gz",
-        sha256 = "934c9ceb552e84577b0faf1e5a2f0450314985b4d8712b2b70717dc679fdc01b",
+        url = "https://github.com/bazelbuild/rules_python/releases/download/0.1.0/rules_python-0.1.0.tar.gz",
+        sha256 = "b6d46438523a3ec0f3cead544190ee13223a52f6a6765a29eae7b7cc24cc83a0",
     )
 
     maybe(