Update Poetry version used for 3.7 test suite and change Requires-Python boundary

[george-zubrienko]

Resolves #452

This PR removes 3.7 from the test suite, thus unlocking our releases since Poetry no longer supports 3.7. In addition, upper bound on minor is removed.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
dataclasses_json
cfg.py	51	4	92%	80, 84–86
core.py	237	10	96%	38–41, 51, 64, 66, 81, 83, 169, 197
mm.py	202	30	85%	33–36, 42–45, 53–56, 62–65, 88, 161–162, 167, 171, 175, 180, 184, 188, 196, 202, 207, 216, 221, 226, 235, 244–251
stringcase.py	25	3	88%	59, 76, 97
undefined.py	143	2	99%	24, 38
utils.py	131	36	73%	12–25, 45–50, 61–65, 75, 100–101, 109–110, 125–133, 163, 182, 207
tests
entities.py	234	3	99%	22, 234, 240
test_annotations.py	81	42	48%	50–67, 78–102, 106–122
test_api.py	142	4	97%	88, 99, 139–140
test_str_subclass.py	22	1	95%	9
test_union.py	98	10	90%	87–94, 108–115
TOTAL	2508	145	94%

Tests	Skipped	Failures	Errors	Time
294	3 💤	0 ❌	0 🔥	7.071s ⏱️

[george-zubrienko]

@lidatong need your help here to remove a required check on 3.7 :)

[george-zubrienko]

I'll see if I can simply skip the 3.7

[george-zubrienko]

@matt035343 I fixed this by changing the Poetry version used in 3.7 check, until we get 3.7 removed from required checks :) Please take a look!