Allow polling TX cancel

Returning nb::Result allows use of nb::block! and should make the cancellation API easier to use.

[x] The code compiles without errors or warnings.
[x] All tests pass and in the best case you also added new tests.
[x] cargo +stable fmt was run.
[x] cargo +stable clippy yields no warnings.
[ ] ~Your changes were added to the CHANGELOG.md in the proper section.~
[x] You add a description of your work to this PR.
[x] You added proper docs (in code, rustdoc and README.md) for your newly added features and code.

GrepitAB / mcan