Purge support for python 2 to clean up code, remove six dependency

[rly]

This PR is not high priority by any means, but since I had started refactoring, I might as well finish the job. To clean up the code and remove the dependency on the six package, I purged all uses of six, e.g., text_type, binary_type, string_types, integer_types, raise_from, with_metaclass, etc.

Also:

• removed any imports of future
• updated .gitignore to use absolute paths which prevents my code editor from ignoring the src/hdmf/build/ directory.
• use python 3 style classes (implicit inheritance from object) and super()

[codecov]

Codecov Report

Merging #213 into dev will decrease coverage by 0.07%.
The diff coverage is 73.49%.

@@            Coverage Diff             @@
##              dev     #213      +/-   ##
==========================================
- Coverage   70.46%   70.38%   -0.08%     
==========================================
  Files          31       31              
  Lines        5935     5920      -15     
  Branches     1390     1389       -1     
==========================================
- Hits         4182     4167      -15     
  Misses       1322     1322              
  Partials      431      431

Impacted Files	Coverage Δ
src/hdmf/monitor.py	0% <0%> (ø)	⬆️
src/hdmf/spec/write.py	77.85% <100%> (-0.16%)	⬇️
src/hdmf/query.py	57.27% <100%> (-0.39%)	⬇️
src/hdmf/common/io/table.py	55.55% <100%> (ø)	⬆️
src/hdmf/utils.py	92.21% <100%> (ø)	⬆️
src/hdmf/backends/io.py	96.77% <100%> (-0.11%)	⬇️
src/hdmf/array.py	51.85% <100%> (-0.36%)	⬇️
src/hdmf/spec/catalog.py	92.07% <100%> (ø)	⬆️
src/hdmf/data_utils.py	90.74% <100%> (-0.07%)	⬇️
src/hdmf/container.py	68.36% <100%> (-0.12%)	⬇️
... and 10 more

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[rly]

@ajtritt can we merge this? this will make the diff for my dimension scales PR significantly easier to follow. thanks