Decrease discovery overhead

[bdraco]

Attempt at reducing the discovery overhead

Before

After -- all the time creating the ssl context is gone:

[emontnemery]

The reason for using urllib directly instead of requests is that the latter did not accept the certificates presented by some chromecasts.
Are we sure this won't be a problem?
Can't we keep the SSL session around without going back to requests?

Edit:
I misremembered the issue, the problem is that when using requests we get these unfilterable warnings in the log:

$ python3 media_example.py --cast "Lab" --known-host=192.168.0.10
/mnt/d/development/github/pychromecast_fork/venv/lib/python3.8/site-packages/urllib3/connectionpool.py:1013: InsecureRequestWarning: Unverified HTTPS request is being made to host '192.168.0.10'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
  warnings.warn(
/mnt/d/development/github/pychromecast_fork/venv/lib/python3.8/site-packages/urllib3/connectionpool.py:1013: InsecureRequestWarning: Unverified HTTPS request is being made to host '192.168.0.10'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
  warnings.warn(
/mnt/d/development/github/pychromecast_fork/venv/lib/python3.8/site-packages/urllib3/connectionpool.py:1013: InsecureRequestWarning: Unverified HTTPS request is being made to host '192.168.0.10'. Adding certificate verification is strongly advised. See: https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
  warnings.warn(

Related PRs:

• Update local http API, re-add get_multizone_status #437 was using requests, but with a hack to disable warnings which would affect all users of requests / urllib3
• Use urllib.request instead of requests #443 moved to urllib to not have to disable warnings globally

The idea in the PR is great, but we should stick with urllib.

[emontnemery]

I attempted to reuse the SSL context in #485, does that work?

[bdraco]

Closing in favor of #485