Release HDMF 2.0

[rly]

This branch and PR collects upcoming changes for HDMF 2.0. See a summary of changes in CHANGELOG.md. This branch should pull changes from dev regularly.

When ready to merge, merge without squashing commits or merge with rebase to remove extraneous merge commits.

[codecov]

Codecov Report

Merging #382 into dev will increase coverage by 2.67%.
The diff coverage is 88.33%.

@@            Coverage Diff             @@
##              dev     #382      +/-   ##
==========================================
+ Coverage   76.26%   78.93%   +2.67%     
==========================================
  Files          33       33              
  Lines        6809     6922     +113     
  Branches     1499     1518      +19     
==========================================
+ Hits         5193     5464     +271     
+ Misses       1214     1067     -147     
+ Partials      402      391      -11     

Impacted Files	Coverage Δ
src/hdmf/build/builders.py	87.89% <ø> (-0.03%)	⬇️
src/hdmf/backends/hdf5/h5_utils.py	81.25% <78.57%> (+14.82%)	⬆️
src/hdmf/build/objectmapper.py	80.07% <83.78%> (+5.40%)	⬆️
src/hdmf/backends/hdf5/h5tools.py	78.33% <86.15%> (+9.76%)	⬆️
src/hdmf/build/manager.py	75.36% <96.29%> (+0.92%)	⬆️
src/hdmf/backends/io.py	98.07% <100.00%> (+0.77%)	⬆️
src/hdmf/container.py	73.71% <100.00%> (+1.42%)	⬆️
src/hdmf/spec/spec.py	75.12% <0.00%> (+0.99%)	⬆️
src/hdmf/spec/write.py	80.53% <0.00%> (+1.34%)	⬆️
... and 4 more

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[rly]

HDMF 2.0 is ready to release after this PR is merged. This PR encompasses #381, #388, #401, and minor updates to

• docs/source/export.rst
• CHANGELOG.md
• requirements files
• docs/source/conf.rst

Note:
The latest versions of numpy (1.19), scipy (1.5), and pandas (1.0) no longer support Python 3.5 so an earlier version is instead pinned in requirements.txt
https://numpy.org/devdocs/release/1.19.0-notes.html
https://docs.scipy.org/doc/scipy/reference/release.1.5.0.html
https://pandas.pydata.org/pandas-docs/stable/whatsnew/v1.0.0.html#increased-minimum-version-for-python

Python 3.5 is also reaching end of life on 2020-09-13
https://devguide.python.org/#status-of-python-branches

We do not yet rely on the new features in these versions of numpy, scipy, and pandas, but we should consider dropping Python 3.5 support soon to 1) access these features and bug fixes and 2) speed up CI.

[rly]

Also, I increased code coverage to 78.9%! :D