Lint code in activestorage/ and add flake8 test at push

[valeriupredoi]

Description

Related to #231

I linted all the code in the main source activestorage/ and added a flake8 test at push.
@davidhassell I'd like a review on this one please, when you have a minute (ie am not gonna barge-merge it like other PRs that were purely functional), the linting is via black, but am not popping black in just yet, I think I'll let flake8 detect any incoming issues for now. What do you think?

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

❌ Patch coverage is 88.18182% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.98%. Comparing base (133537d) to head (4c3e3fd).
⚠️ Report is 39 commits behind head on main.

Files with missing lines	Patch %	Lines
activestorage/active.py	77.77%	10 Missing ⚠️
activestorage/dummy_data.py	97.22%	1 Missing ⚠️
activestorage/hdf2numcodec.py	66.66%	1 Missing ⚠️
activestorage/reductionist.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
+ Coverage   85.20%   85.98%   +0.77%     
==========================================
  Files           7        7              
  Lines         649      642       -7     
==========================================
- Hits          553      552       -1     
+ Misses         96       90       -6     

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[valeriupredoi]

Ive decided to skip black -ing the code, too specific a formatter for now, so will just pylint with yapf instead, minimal approach 🤏

[valeriupredoi]

cov variation is OK - we're covered by "slow" tests that are skipped - the actual absolute coverage increases by 0.8%
@davidhassell the actual code looks pretty good, am gonna merge 🍻