Replace uses of the Unicode trait with Str

[mdickinson]

This PR replaces all uses of the deprecated Unicode trait type with Str.

[mdickinson]

I was jumping the gun a bit here. We can't do this until after we've dropped Python 2 support.

[rahulporuri]

This PR is ready to go in now because the upcoming release will not support Python 2.

[codecov-io]

Codecov Report

Merging #123 into master will decrease coverage by 1.53%.
The diff coverage is 26.92%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   41.67%   40.13%   -1.54%     
==========================================
  Files         240      240              
  Lines        8912     8909       -3     
  Branches     1105     1161      +56     
==========================================
- Hits         3714     3576     -138     
- Misses       5044     5189     +145     
+ Partials      154      144      -10     

Impacted Files	Coverage Δ
...ools/appscripting/action/start_recording_action.py	0.00% <0.00%> (ø)
...tools/appscripting/action/stop_recording_action.py	0.00% <0.00%> (ø)
apptools/appscripting/i_script_manager.py	0.00% <0.00%> (ø)
apptools/appscripting/script_manager.py	0.00% <0.00%> (ø)
apptools/help/help_plugin/help_plugin.py	0.00% <0.00%> (ø)
apptools/help/help_plugin/help_submenu_manager.py	0.00% <ø> (ø)
apptools/help/help_plugin/preferences_pages.py	0.00% <ø> (ø)
apptools/io/file.py	82.03% <ø> (ø)
apptools/logger/agent/quality_agent_view.py	0.00% <ø> (ø)
apptools/logger/custom_excepthook.py	0.00% <ø> (ø)
... and 121 more

---

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[kitchoi]

This PR is ready to go in now because the upcoming release will not support Python 2.

I am not sure what sequence of events we are going after here for #120.
e.g. I'd think both setup.py and etstool.py need to be updated to prevent Python 2 from being used before changes like this PR goes ahead.

[rahulporuri]

I think we can merge this after #190 is merged (after fixing merge conflicts of course).

[kitchoi]

LGTM

[kitchoi]

I will be resolving the conflicts...

[mdickinson]

We could go further and drop the u prefixes, but that's probably best done in a separate PR.

[mdickinson]

@kitchoi Thank you for the changes! I can't officially approve because I opened the PR in the first place, but LGTM.

[aaronayres35]

LGTM

[kitchoi]

Thank you all!