put all the various requirements constellations into txt files

[glyph]

fixes #551

also removes python3.6 support from the test matrix because there is a bunch of tedious version-constraint work to do here in order to get it to work, and I don't feel like bothering for a dead-for-half-a-year version of Python: https://endoflife.date/python

[codecov]

Codecov Report

Merging #552 (37338e4) into master (79f9be4) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   98.86%   99.04%   +0.18%     
==========================================
  Files          44       44              
  Lines        3880     3881       +1     
  Branches      245      357     +112     
==========================================
+ Hits         3836     3844       +8     
+ Misses         30       23       -7     
  Partials       14       14              

Impacted Files	Coverage Δ
src/klein/_plating.py	100.00% <ø> (ø)
src/klein/_app.py	100.00% <100.00%> (ø)
src/klein/_resource.py	96.52% <100.00%> (+4.91%)	⬆️
src/klein/test/test_resource.py	98.34% <100.00%> (ø)

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[glyph]

@wsanchez @adiroiban CI is completely broken and deps are wildly out of date, so I'm just merging this to unblock everything. I'd love a post-hoc review though, and I'd be happy to make updates to trunk once we're back in a green starting state.

[twm]

@glyph The move to .txt files in the requirements directory is for compatibility with some Dependabot heuristic, correct?

[glyph]

@twm Yes. Although it's an idiom that predates dependabot, and it's easier to maintain with tools like pur and requires.io as well, though, so it's not just for Dependabot.