Add oldest & minimal CI configurations.

[bdice]

Description

• Adds "oldest" CI configuration. Python 3.6 is no longer supported by the latest versions of our testing dependencies.
• Adds "minimal" CI configuration with Python 3.9, to ensure that our package's hard requirements are enough to pass tests (and ensure that users who don't have the optional dependencies installed can still use the package's core features).
• Re-organizes all the CI requirements files into a subdirectory called requirements.

The "oldest" changes are discussed in PR #460, which this supersedes. I talked through the process of this PR with @atravitz and @kidrahahjo during signac office hours.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #474 (d2bceb7) into master (d4f2121) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
+ Coverage   77.08%   77.18%   +0.10%     
==========================================
  Files          42       42              
  Lines        5768     5768              
  Branches     1129     1129              
==========================================
+ Hits         4446     4452       +6     
+ Misses       1034     1030       -4     
+ Partials      288      286       -2     

Impacted Files	Coverage Δ
signac/core/synceddict.py	96.01% <0.00%> (+1.32%)	⬆️
signac/core/json.py	84.00% <0.00%> (+12.00%)	⬆️

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[kidrahahjo]

I will make the same changes for flow.