ci: fix free-threading issue in test_coroutine

[davidhewitt]

Three tests in test_coroutine all import the __main__ module and then execute some code which defines a symbol main().

This is problematic on the free-threaded build because the definition of def main() and then the following call to asyncio.run() can race with the other tests overwriting the main symbol with their own implementation of it.

This can lead to the wrong code getting called in the tests & unexpected failures!

cc @ngoldbaum - no issues in CPython or PyO3 to worry about, just a flaky test suite 😂

[ngoldbaum]

The fix makes sense to me. I'm pretty sure the codecov failure is spurious.

Teeny style cleanup suggestion - probably for a followup - is to edit all these so they call the Python token py instead of gil - gil is unidiomatic compared with the rest of the codebase.

± rg "with_gil\(\|gil"
src/impl_/coroutine.rs
53:        Python::with_gil(|gil| {
90:        Python::with_gil(|gil| {

src/coroutine/waker.rs
44:        Python::with_gil(|gil| {

guide/src/async-await.md
69:        Python::with_gil(|gil| {

tests/test_coroutine.rs
35:    Python::with_gil(|gil| {
61:    Python::with_gil(|gil| {
96:    Python::with_gil(|gil| {
115:    Python::with_gil(|gil| {
129:    Python::with_gil(|gil| {
138:    Python::with_gil(|gil| {
177:    Python::with_gil(|gil| {
209:    Python::with_gil(|gil| {
237:    Python::with_gil(|gil| {
287:    Python::with_gil(|gil| {
345:    Python::with_gil(|gil| {