mypy

a8ab85da - [mypyc] Improve access to generated C on test failures and document this (#18476)

[mypyc] Improve access to generated C on test failures and document this (#18476) Now the generated C files for the first mypyc run test failure in a pytest session will be copied to the `.mypyc_test_output` directory, and this will be indicated in the test output. This is a convenience feature to help in the common scenario where all test failures have the same root cause, so any single output is sufficient for debugging. Document this and `--mypyc-showc`, which allows showing generated C for every test failure. The latter is too verbose to be enabled by default.