RF,ENH(TST): future proof testing of git annex version upgrade + test annex init on all supported versions

[yarikoptic]

Originally this test was crafted when 6 was still among supported versions.
We have gone past that point now, minimal supported version of git-annex
(from 202003) supports only 8, and current ones default to 8 but support also
9 and 10. I have RF and extended test to to remain valid "forever" and test
that we can init git-annex of any supported version

[codecov]

Codecov Report

Merging #6880 (ca7460f) into maint (cefecae) will increase coverage by 0.95%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            maint    #6880      +/-   ##
==========================================
+ Coverage   90.25%   91.21%   +0.95%     
==========================================
  Files         354      354              
  Lines       46113    46123      +10     
==========================================
+ Hits        41621    42070     +449     
+ Misses       4492     4053     -439     

Impacted Files	Coverage Δ
datalad/support/tests/test_annexrepo.py	97.82% <100.00%> (+0.07%)	⬆️
datalad/api.py	86.66% <0.00%> (-8.08%)	⬇️
datalad/tests/utils.py	66.76% <0.00%> (+9.73%)	⬆️
datalad/tests/test_tests_utils.py	92.34% <0.00%> (+92.34%)	⬆️

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[yarikoptic]

docs known, so likely unrelated. travis - one known and one annoying recent filed #6887 . These two tests passed just fine everywhere

$> git grep test_repo_version | nl | tail -n 2
    43	ca7460f/travis-14312-success/8.txt:../datalad/support/tests/test_annexrepo.py::test_repo_version_upgrade PASSED [ 59%]
    44	ca7460f/travis-14312-success/8.txt:../datalad/support/tests/test_annexrepo.py::test_repo_version_supported[8] PASSED [ 59%]

$> git grep test_repo_version | grep -v PASSED