zulip/README: Specify the required Python version.
We recently dropped support for Python versions less than 3.5.
Since the README is also used as the project description on our
PyPI homepage, we need to be more explicit about what version of
Python our bindings require.
#### Dependencies
The [Zulip API](https://zulipchat.com/api) Python bindings require the
-following Python libraries:
+following dependencies:
+* **Python (version >= 3.5)**
* requests (version >= 0.12.1)
* six
* typing (version >= 3.5.2.2)
