Apply uniform code style

[khaeru]

Following the example of other, high-quality scientific Python packages, this PR uses some automatic tools to create a uniform code style: isort, black, and mypy. (Another tool, flake8, was already applied as part of #28.)

It also:

• adds these tools to the GitHub Actions continuous integration workflow, to ensure that all new code in PRs is checked for quality/compliance, and
• adds them to the development guidelines.

[codecov[bot]]

Codecov Report

Merging #29 into master will increase coverage by 0.16%. The diff coverage is 47.76%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   50.92%   51.08%   +0.16%     
==========================================
  Files          34       34              
  Lines        1467     1472       +5     
==========================================
+ Hits          747      752       +5     
  Misses        720      720              

Impacted Files	Coverage Δ
item/historical/legacy.py	0.00% <0.00%> (ø)
item/model/bp.py	0.00% <0.00%> (ø)
item/model/cli.py	0.00% <0.00%> (ø)
item/model/eia.py	0.00% <0.00%> (ø)
item/model/eppa5.py	0.00% <0.00%> (ø)
item/model/exxonmobil.py	0.00% <0.00%> (ø)
item/model/get.py	0.00% <0.00%> (ø)
item/model/itf.py	0.00% <0.00%> (ø)
item/model/message.py	0.00% <0.00%> (ø)
item/tests/conftest.py	0.00% <0.00%> (ø)
... and 24 more

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[khaeru]

The "codecov/patch" check failure here is spurious.