Fix dates (without times) and times (without dates)

[novas0x2a]

I found two separate but related problems, the first is a date without a time (fixed by adding datetime.date to the JSONDateTimeEncoder; the second a time without a date, which yaml refers to as a sexagesimal. YAML 1.1 specifies that these parse as base 60 integers, and can't easily be round-tripped. I fixed that by switching to an api-compatible yaml loader that supports yaml 1.2, which eliminates sexagesimals among other things.

[codecov-io]

Codecov Report

Merging #10 into master will not change coverage.
The diff coverage is 75%.

@@           Coverage Diff           @@
##           master      #10   +/-   ##
=======================================
  Coverage   84.72%   84.72%           
=======================================
  Files           2        2           
  Lines          72       72           
=======================================
  Hits           61       61           
  Misses         11       11

Impacted Files	Coverage Δ
yq/__init__.py	84.5% <75%> (ø)	⬆️

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[kislyuk]

Thank you for reporting. I'm not ready to switch to ruamel.yaml though. We will need a different solution for now.

[kislyuk]

Thank you again for reporting and working on this, @novas0x2a. I finally got around to fixing the issue with bare dates. As I mentioned though, I'm not ready to switch to ruamel.yaml yet - I'd like to wait a few more months for https://github.com/yaml/pyyaml to show signs of life. Until then, I've added a test case marked as an expected failure.