chore: limit lint runs to master pushes and PRs

[sabard]

As tests were running twice per commit in PRs, so is linting. This change prevents that.

Shouldn't make a difference so long as the default branch is protected from merging without a PR and people are using pre-commit.

[codecov]

Codecov Report

Base: 96.36% // Head: 96.39% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (d24153f) compared to base (d3a4320).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #382      +/-   ##
==========================================
+ Coverage   96.36%   96.39%   +0.03%     
==========================================
  Files           9        9              
  Lines         908      916       +8     
==========================================
+ Hits          875      883       +8     
  Misses         33       33              

Impacted Files	Coverage Δ
graphene_sqlalchemy/converter.py	95.83% <0.00%> (+0.01%)	⬆️
graphene_sqlalchemy/utils.py	96.09% <0.00%> (+0.22%)	⬆️

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