Remove resolver argument from Cargo.toml

[mtreinish]

When building rust-numpy with the rust/cargo 1.48 packaged on Debian
bullseye the build fails because in Cargo 1.48 the resolver option was a
nightly only feature and Cargo errors. This feature wasn't stabilized
until 1.51. I think we aren't catching this in the MSRV jobs because
1.41 is old enough that the resolver option wasn't known.

[adamreichold]

Shouldn't this be a full revert of 7817be6 then? The original PR had

This saves a bunch of unnecessary dependencies when using rust-numpy as a dependency.

as its motivation. So this does sound like an optimisation, not a necessity. But to be sure, cc @m-ou-se ?

[m-ou-se]

You can keep the dev-dependency/dependency split, but only remove the resolver = "2" part. (Exactly as this PR is doing.) Then, a project that uses pyo3-numpy can still pick resolver = "2" to benefit from the reduced number of dependencies and features.

[kngwyu]

Thanks!

You can keep the dev-dependency/dependency split, but only remove the resolver = "2" part. (Exactly as this PR is doing.) Then, a project that uses pyo3-numpy can still pick resolver = "2" to benefit from the reduced number of dependencies and features.

Sounds reasonable.