Make urllib3 optional.

[tomchristie]

Follow up to #963.

Let's ensure that the urllib3 dependency is optional.

• Make sure we don't have it anymore in setup.py.
• Make sure it's listed as optional in the REAEDME/homepage.
• Make sure that the urllib3.py module passes gracefully on ImportError, and checks the package exists in URLLib3Transport.__init__().

[yeraydiazdiaz]

The last point is already covered, unless you have a different implementation in mind?

[tomchristie]

Ah fantastic!

[tomchristie]

Have also confirmed that it's not in setup.py, so all we have is...

• Docs tweak in the advanced section, noting to make sure urllib3 is installed.
• In README.md and doc/index.md update the dependencies listing. (I guess move urllib3 to just above brotlipy as the other optional, and have httpcore with h11, h2 hanging off as subdependancies, like so...

---

• httpcore - The default transport implementation for httpx.
  • h11 - HTTP/1.1 support.
  • h2 - HTTP/2 support.
• certifi - SSL certificates.
• chardet - Fallback auto-detection for response encoding.
• hstspreload - determines whether IDNA-encoded host should be only accessed via HTTPS.
• idna - Internationalized domain name support.
• rfc3986 - URL parsing & normalization.
• urllib3 - Support for httpx.URLLib3Transport(). (Optional)
• brotlipy - Decoding for "brotli" compressed responses. (Optional)