Handle sys.stdout.encoding being None in get_style()

[devdanzin]

If sys.stdout is a StringIO instance, sys.stdout.encoding will be None and wily.helper.get_style() will error out due to trying to call .lower() on it. This PR adds a check to handle that case.

Found while updating #199, due to a test that mocked stdout in this way.

[codecov-commenter]

Codecov Report

Merging #204 (048a5db) into master (d501d01) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #204   +/-   ##
=======================================
  Coverage   95.42%   95.43%           
=======================================
  Files          24       24           
  Lines        1269     1270    +1     
  Branches      287      287           
=======================================
+ Hits         1211     1212    +1     
  Misses         33       33           
  Partials       25       25           

Files Changed	Coverage Δ
src/wily/helper/__init__.py	100.00% <100.00%> (ø)