Fix BigInt export

[erikwrede]

Fixes #1436

[erikwrede]

@ulgens 🙂 👋

[codecov]

Codecov Report

Merging #1456 (ae7fb56) into master (355601b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1456   +/-   ##
=======================================
  Coverage   95.44%   95.44%           
=======================================
  Files          49       49           
  Lines        1582     1582           
=======================================
  Hits         1510     1510           
  Misses         72       72           

Impacted Files	Coverage Δ
graphene/__init__.py	100.00% <ø> (ø)
graphene/types/__init__.py	100.00% <100.00%> (ø)

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[ulgens]

@erikwrede The change looks fine. Are we testing this stuff anywhere? If so, can you please update the related tests? Thank you.

[erikwrede]

Thank you!
I haven't found any relevant test for exports. Might be a good idea to add a test for that in a future PR!