Move `warn_incomplete_stub` to global options (#705)
Move `warn_incomplete_stub` to global options (#705)

A follow-up to #698.

This suppresses the following warning from mypy, by making the flag
apply globally rather than to a specific module.

```
pyproject.toml: [module = "nacl.bindings.crypto_aead"]: Per-module sections should only specify per-module flags (warn_incomplete_stub)
pyproject.toml: [module = "nacl.encoding"]: Per-module sections should only specify per-module flags (warn_incomplete_stub)
pyproject.toml: [module = "nacl.exceptions"]: Per-module sections should only specify per-module flags (warn_incomplete_stub)
pyproject.toml: [module = "nacl.utils"]: Per-module sections should only specify per-module flags (warn_incomplete_stub)
```
pyca/pynacl,pyca/pynacl,pyca/pynacl,pyca/pynacl,pyca/pynacl