Fix type mismatches in traceplot.py.

[obi1kenobi]

Description

Fixing a couple more mypy errors, closing in toward #1496:

arviz/plots/traceplot.py:23: error: Incompatible default for argument "circ_var_units" (default has type "str", argument has type "bool")  [assignment]
arviz/plots/traceplot.py:163: error: Incompatible types in assignment (expression has type "bool", variable has type "Optional[str]")  [assignment]

Checklist

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

[codecov]

Codecov Report

Merging #1497 (fa86ac7) into master (fff2b2f) will increase coverage by 1.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1497      +/-   ##
==========================================
+ Coverage   90.37%   91.92%   +1.54%     
==========================================
  Files         105      105              
  Lines       11246    11246              
==========================================
+ Hits        10164    10338     +174     
+ Misses       1082      908     -174     

Impacted Files	Coverage Δ
arviz/plots/traceplot.py	95.12% <100.00%> (ø)
arviz/data/io_emcee.py	98.33% <0.00%> (+2.50%)	⬆️
arviz/data/io_pystan.py	96.00% <0.00%> (+32.50%)	⬆️

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