Resolve a few straightforward mypy type errors.

[obi1kenobi]

Description

This PR is only about improving type hint coverage and quality. It contains no new features and makes no substantive changes to existing functionality, so it should be fully covered by existing tests. This is similar in spirit to #1397 and #1398.

In particular, this PR:

• Applies a mypy-suggested type hint to a test helper that requires it in order to type-check.
• Applies # type: ignore suppressions to a few instances of a known and still-unresolved mypy issue mypy bug with try/except conditional imports python/mypy#1153

The following mypy-reported errors are resolved as a result:

arviz/data/io_pystan.py:20: error: Name 'json' already defined (by an import)  [no-redef]
arviz/data/io_json.py:9: error: Name 'json' already defined (by an import)  [no-redef]
arviz/data/base.py:15: error: Name 'json' already defined (by an import)  [no-redef]
arviz/tests/helpers.py:259: error: Incompatible return value type (got "List[str]", expected "List[Union[str, Tuple[str, str]]]")  [return-value]
arviz/tests/helpers.py:259: note: "List" is invariant -- see http://mypy.readthedocs.io/en/latest/common_issues.html#variance
arviz/tests/helpers.py:259: note: Consider using "Sequence" instead, which is covariant
arviz/tests/helpers.py:259: note: Perhaps you need a type annotation for "failed_attrs"? Suggestion: "List[Union[str, Tuple[str, str]]]"

Additionally, a similar mypy type error related to a conditional ujson import that would be uncovered if #1491 merges is also addressed here.

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)

[codecov]

Codecov Report

Merging #1492 (a27b0fa) into master (2d202de) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1492   +/-   ##
=======================================
  Coverage   91.97%   91.97%           
=======================================
  Files         105      105           
  Lines       11239    11239           
=======================================
  Hits        10337    10337           
  Misses        902      902           

Impacted Files	Coverage Δ
arviz/data/base.py	97.70% <0.00%> (ø)
arviz/data/inference_data.py	85.81% <0.00%> (ø)
arviz/data/io_json.py	66.66% <0.00%> (ø)
arviz/data/io_pystan.py	96.00% <0.00%> (ø)

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[canyon289]

Thank you for the contribution!