Fix tests without num-bigint feature

[cjwatson]

cargo test --no-default-features and cargo test --no-default-features -F python failed due to various tests that required the num-bigint feature. Annotate these appropriately.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.92%. Comparing base (dbf0c52) to head (bd37d19).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   88.92%   88.92%           
=======================================
  Files          13       13           
  Lines        2195     2195           
  Branches     2195     2195           
=======================================
  Hits         1952     1952           
  Misses        148      148           
  Partials       95       95           

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[codspeed-hq]

CodSpeed Performance Report

Merging #169 will improve performances by 12.03%

_{Comparing cjwatson:num-bigint-tests (bd37d19) with main (dbf0c52)}

Summary

⚡ 1 improvements
✅ 72 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	cjwatson:num-bigint-tests	Change
⚡	unicode_jiter_iter	8.6 µs	7.7 µs	+12.03%

[davidhewitt]

Thanks 👍