feat(python): produce code coverage from a unit test

[alexeagle]

Ask Aspect workflows to upload the coverage data, though I'm expecting that's going to fail until I add a Codecov token to our GHA secrets.

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Changes are visible to end-users: no

Test plan

• New test cases added

[aspect-workflows]

Test

⚠️ Buildkite build #459 failed.

//speller/data_driven_tests:test-001 failed to build

not all outputs were created or valid

//speller/lookup:lookup-test failed to build

not all outputs were created or valid

//speller/data_driven_tests:test-002 failed to build

not all outputs were created or valid

//speller/greeting:greeting-test failed to build

not all outputs were created or valid

💡 To reproduce the build failures, run

bazel build //speller/data_driven_tests:test-001 //speller/lookup:lookup-test //speller/data_driven_tests:test-002 //speller/greeting:greeting-test

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