Fix missing std::sync::atomic::Ordering import for targets without atomic64

[rara64]

This PR resolves an issue introduced in commit 5589b17 where builds for targets without 64-bit atomics will fail with an undeclared type error.

std::sync::atomic::Ordering should be imported unconditionally in the use std::{ ... } block.

Build output from armel (32-bit architecture):

Compiling pyo3 v0.28.0 (/pyo3)
Compiling pyo3-macros v0.28.0 (/pyo3/pyo3-macros)
error[E0433]: failed to resolve: use of undeclared type `Ordering`
   --> src/impl_/pymodule.rs:125:21
    |
125 |                     Ordering::SeqCst,
    |                     ^^^^^^^^ use of undeclared type `Ordering`
    |
help: consider importing one of these enums
    |
3   + use std::cmp::Ordering;
    |
3   + use std::sync::atomic::Ordering;
    |
3   + use portable_atomic::Ordering;
    |

error[E0433]: failed to resolve: use of undeclared type `Ordering`
   --> src/impl_/pymodule.rs:126:21
    |
126 |                     Ordering::SeqCst,
    |                     ^^^^^^^^ use of undeclared type `Ordering`
    |
help: consider importing one of these enums
    |
3   + use std::cmp::Ordering;
    |
3   + use std::sync::atomic::Ordering;
    |
3   + use portable_atomic::Ordering;
    |

For more information about this error, try `rustc --explain E0433`.
error: could not compile `pyo3` (lib) due to 2 previous errors

Affected releases:

• PyO3 0.28.0

[alex]

Ooops, I think there's some cfg twiddling needing

[rara64]

Ooops, I think there's some cfg twiddling needing

Yep, I'm on it.