Singleton and classproperty support

[netsettler]

The original purpose of thie PR was to add some substrate for use by David in foursight-core and probably elsewhere in 4dn-cloud-infra. We've had a number of needs for both the classproperty and Singleton support.

In the process, I brought misc_utils coverage up to 100% and fixed a few tiny and probably inconsequential bugs that I noticed when adding those tests.

• In misc_utils:

  • New decorator @classproperty

  • New decorator @classproperty_cached

  • New decorator @classproperty_cached_each_subclass

  • New class Singleton. Users of SingletonManager might prefer this, but we'll continue to support both. (No deprecation for now.)

  • In function is_valid_absolute_uri, better handling of argument type errors.

  • For CachedField:

  • Added a handler for __str__ that returns useful information, which can also be used for __repr__.

  • Fixed handler for __repr__ to return a properly executable expression (shared with __str__).

  • Improved test coverage by adding tests for some parts of the code that were not previously tested.

• In qa_utils:

  • New class MockId for mocking the id function in a predictable way.

  • Adjust MOCK_QUEUE_URL_PREFIX to use a mocked URL that looks more like modern AWS url, where queue.amazonaws.com has been replaced by sqs.us-east-1.amazonaws.com.

[coveralls]

Pull Request Test Coverage Report for Build 3495875128

• 88 of 88 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 73.475%

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Totals
Change from base Build 3198667051:	0.5%
Covered Lines:	5781
Relevant Lines:	7868

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💛 - Coveralls