drop 3.7 support

[charles-cooper]

What I did

drop tests for 3.7 since isqrt does not exist in 3.8. note that vyper can still be installed on 3.7 but we no longer officially test it.

How I did it

How to verify it

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #3071 (171041d) into master (e7733e1) will decrease coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3071      +/-   ##
==========================================
- Coverage   88.36%   88.05%   -0.32%     
==========================================
  Files          97       97              
  Lines       10980    10980              
  Branches     2597     2597              
==========================================
- Hits         9703     9668      -35     
- Misses        827      864      +37     
+ Partials      450      448       -2     

Impacted Files	Coverage Δ
vyper/__init__.py	58.82% <0.00%> (-17.65%)	⬇️
vyper/codegen/arithmetic.py	79.48% <0.00%> (-5.13%)	⬇️
vyper/semantics/types/value/numeric.py	80.45% <0.00%> (-4.60%)	⬇️
vyper/builtin_functions/functions.py	89.00% <0.00%> (-1.34%)	⬇️
vyper/utils.py	83.73% <0.00%> (-0.96%)	⬇️
vyper/ir/optimizer.py	97.56% <0.00%> (-0.31%)	⬇️
vyper/ast/nodes.py	93.01% <0.00%> (-0.17%)	⬇️

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