Mock https requests in gist tests

[jacobtomlinson]

Description

Update tests for GitHub Gists to use mocked API calls.

Partial fix for #593

Status

READY

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #602 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #602   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          20     20           
  Lines        1369   1369           
=====================================
  Hits         1369   1369

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