Remove the dependency on tomli to simplify installation

[alex]

Depending on tomli produces circular dependencies when attempting to install from sdists only: pyca/cryptography#6671

[davidhewitt]

Thanks, using cargo metadata is a brilliant solution. Reading the cryptography thread, sorry for the pain caused.

I'll follow up this PR tomorrow morning with a quick refactoring so we only need to call cargo metadata once (we use it in one other place too) and then put this patch release live.

[alex]

Sounds great, thanks!