feat(extension) Replace rust-cpython with pyo3

[Hywan]

This patch replaces the rust-cpython crate by the pyo3 crate.

1. Rust nightly is required to compile the extension (Compile with stable rust PyO3/pyo3#5),
2. pyo3 is more active,
3. Remove support for Python 2 (wasn't working before anyway),
4. Allow to declare getters and __call__: We can now introduce this new API: instance.exports['sum'](1, 2) instead of instance.call('sum', [Value.i32(1), Value.i32(2)]),
5. Simpler error/exception API,
6. Much simpler code: Ease contributions,
7. Faster compilation,
8. Doesn't impact the publishing process (i.e. pushing wheels on pypi),
9. Remove the Shell hack.