[release-0.1] Fix concurrency issue. Require Julia v1.6 and update CI setup.

[IanButterworth]

Fixes #12
Includes #13

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.65%. Comparing base (433285f) to head (1b063a0).
⚠️ Report is 1 commits behind head on release-0.1.

Additional details and impacted files

@@               Coverage Diff               @@
##           release-0.1      #14      +/-   ##
===============================================
- Coverage        97.85%   95.65%   -2.21%     
===============================================
  Files                1        1              
  Lines              140      138       -2     
===============================================
- Hits               137      132       -5     
- Misses               3        6       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.