Remove unittest2 dependency

[rly]

Python 2 will no longer be supported, so we can remove the dependency and usage of unittest2 and traceback2. This helps break up the mega PR #1117

[codecov]

Codecov Report

Merging #1142 into dev will increase coverage by 0.34%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev    #1142      +/-   ##
==========================================
+ Coverage   67.49%   67.83%   +0.34%     
==========================================
  Files          38       38              
  Lines        2301     2279      -22     
  Branches      397      397              
==========================================
- Hits         1553     1546       -7     
+ Misses        679      664      -15     
  Partials       69       69

Impacted Files	Coverage Δ
src/pynwb/validate.py	0% <ø> (ø)	⬆️
src/pynwb/retinotopy.py	100% <100%> (+5.12%)	⬆️
src/pynwb/ophys.py	94.26% <100%> (+1.46%)	⬆️
src/pynwb/misc.py	85.08% <100%> (+1.32%)	⬆️
src/pynwb/file.py	71.37% <100%> (+0.45%)	⬆️
src/pynwb/image.py	93.54% <100%> (+2.77%)	⬆️
src/pynwb/base.py	92% <100%> (+1.37%)	⬆️
src/pynwb/ecephys.py	96.07% <100%> (+1.79%)	⬆️

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