[bazel] add python rules and register python toolchain
[bazel] add python rules and register python toolchain

Currently, Python scripts are referenced across our repository as bazel
files/filegroups, rather than using actualy Python bazel rules.
Moreover, our current bazel configuration did not enforce hermiticity
w.r.t. a python toolchain or packages. By adding Python rules and
registering a toolchain, makes hermetic Python builds possible.

This fixes #11683.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan