Update reqs

[rly]

Update requirements to latest versions and clean up unused requirements, including Python2 backports like unittest2.

This should speed up CI processes and developer environment setup.

[codecov]

Codecov Report

Merging #176 into dev will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              dev     #176      +/-   ##
==========================================
+ Coverage   70.36%   70.42%   +0.05%     
==========================================
  Files          30       30              
  Lines        5916     5907       -9     
  Branches     1382     1382              
==========================================
- Hits         4163     4160       -3     
+ Misses       1322     1316       -6     
  Partials      431      431

Impacted Files	Coverage Δ
src/hdmf/backends/hdf5/h5_utils.py	62.29% <100%> (+0.35%)	⬆️
src/hdmf/data_utils.py	90.59% <100%> (+0.59%)	⬆️
src/hdmf/build/builders.py	85.25% <100%> (+0.55%)	⬆️

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