Add Julia 1.7 to CI matrix

[sostock]

No description provided.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@4515ae0). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #26   +/-   ##
=========================================
  Coverage          ?   95.43%           
=========================================
  Files             ?        6           
  Lines             ?      548           
  Branches          ?        0           
=========================================
  Hits              ?      523           
  Misses            ?       25           
  Partials          ?        0           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4515ae0...9610698. Read the comment docs.