This upstreams a couple trivial Google patches to make rules_python
more compatible with some Google-internal changes.

* Use list(dict) instead of dict.keys(). This is because some bzl files
  are run through a Python-based testing framework, and dict.keys()
  can't be concatenated to a list in Python 3.
* Expand visibility to all of rules_python instead of just the
  python subdirectory. This makes some patches that use internals
  easier to maintain, but also makes the visiblity more in line
  with the rest of the project (where `//:__subpackages__` is
  used for convenience, as it avoids having to change visibility
  frequently, but still prevents public dependencies).