pyo3.bbclass: move from meta-python

pyo3.bbclass: move from meta-python

PyO3 provides Rust bindings for Python, including tools for creating native
Python extension modules. Running and interacting with Python code from a
Rust binary is also supported.

This class sets up the cross-compilation environment.

Export PYO3_CROSS, PYO3_CROSS_LIB_DIR, PYO3_CROSS_INCLUDE_DIR and CARGO_BUILD_TARGET
to inform tools like setuptools-rust we are cross-compiling.

Export RUSTFLAGS so cargo can find 'std' and 'core' for target

Dynamically generate PyO3 config file and export PYO3_CONFIG_FILE
absolute path. This is the trick that finally made pyo3 work.

Signed-off-by: Tim Orling <tim.orling@konsulko.com>