build: Set only lower bounds on core dependencies

[matthewfeickert]

Description

Following @henryiii's and @hynek's advice (for the core dependencies now, we'll get to discussing the ones for the setup.py extras later) only place lower bounds on the core library dependencies, which have all reached maturity in their API.

This was more recently motivated by the fact that Henry pointed out that we'll be excluding use of Click v8.0.0 (when it is released, currently only v8.0.0a1 on PyPI), but there's no reason to want to bound Click to only v7.X or v8.X.

Thanks also to a very good comment from @tacaswell this also reverts the effects of PR #1378:

I think this is another library vs application difference. Your users are adults who are responsible for keeping their own house in order 😉 It is not a random libraries job to keep their systems up-to-date and secure (that falls to the end users and the packagers).

I think this is also a good case for why libraries should be as loose as possible with their version pinning as say other library was being super picky. Say some other library was being strict to some version range of pyyaml < 5.4 (for whatever reasons). Bumping your minimum to >= 5.4 with good intentions just drove someone to have unsatisfyable constraints.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Place only lower bounds (>=) on all core dependencies in setup.cfg
* Update lower bounds for core dependencies
   - Lower bounds established through empirical tests in CI with minimum supported dependencies workflow
* Effectively reverts PR #1378

[codecov]

Codecov Report

Merging #1382 (04b4732) into master (cfc4b6a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1382   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files          63       63           
  Lines        3808     3808           
  Branches      538      538           
=======================================
  Hits         3714     3714           
  Misses         55       55           
  Partials       39       39           

Flag	Coverage Δ
contrib	24.18% <ø> (ø)
unittests	97.53% <ø> (ø)

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[matthewfeickert]

This is still a Draft PR as I want to go through and test if these lower bounds that we have can be safely pushed down any lower.