chore(typing): clean up some of the workrounds from the initial PR

[ThosRTanner]

Why is this needed?

This addresses some of the issues spotted when adding type checking but left until later due to the extent of the initial pull request.

Going to do more pull requests based from this later, but this one is probably big enough as it is.

Proposed Changes

• Clean up the kazoo client __init__ method to setup up the retry handlers and hosts lists more cleanly
• Refactor the common code in set_hosts and __init__ to simplify both to eventually make it possible to specify the chroot separately to the __init__ method and not have to keep passing it if you change the host list.
• Change hosts/collect_hosts to always return an empty string if no chroot is specified.
• Remove some tests because what was being tested would not have passed mypy were it not for the way self.assertRaises handles the parameter list.
• Clean up the fileobj_to_fd in handlers/utils as the checks aren't necessary which simplifies the place it is used
• Move Create2 in serialization.py so we don't have to quote the name for the Transaction types. Although it means they're not in numerical order, there were odd gaps in the numbering, so it was of questionable help.
• Remove incorrect comment in recipe/cache.py
• Simplify the code in recipe/counter.py to avoid type casting
• Change the _Watch class in recipe/lock.py to immediately start counting, and avoid some messy type casting
• Refactor test_cache.py to use a context handler for the TreeCache, making it generally easier to understand
• Remove unnecessary type casting in test_selectors_select.py as stdout admits it has a fileno method, unlike the result of os.popen
• Replace wrapper function in test_retry with functools.partial
• Use importlib.utils to determine if an import is available rather than using import x # noqa
• Reduce number of places where typing.Any is used and needn't be.
• Replace logger.warn calls with logger.warning

Does this PR introduce any breaking change?

If anyone directly calls kazoo.hosts.collect_hosts it will no longer return None for the chroot. It will return "" in that situation. But that isn't part of the published API.

Codecov report

Codecov is still a few releases behind HEAD, so it's still reporting on the nominal missing coverage reported there (which makes it quite difficult to trust...)

[codecov]

Codecov Report

❌ Patch coverage is 98.93617% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.56%. Comparing base (e894342) to head (09821ec).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
kazoo/protocol/connection.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #801      +/-   ##
==========================================
- Coverage   96.65%   95.56%   -1.10%     
==========================================
  Files          27       27              
  Lines        3554     3830     +276     
==========================================
+ Hits         3435     3660     +225     
- Misses        119      170      +51     

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