PEP 850: Wheel Variants: Providers

This is the second PEP split off PEP 817. Its focus is on how variant
properties are governed and how their compatibility is determined. This
is done either via static feature and feature value compatibility lists
specified in the variant metadata, or by querying opt-in plugins that
are Python packages, but can also be vendored or reimplemented by the
tools. A static list can also be provided in place of querying the
plugins.  Additionally, an ABI Dependency Variant Provider is defined to
facilitate builds of the same package against different dependency
versions.

Compared to PEP 817, the provider metadata has been largely simplified
by removing all the bits deemed not strictly necessary, and adjusted for
the changes in PEP 825. The provider plugins have been made opt-in (with
provisions for tools to make some of them opt-out). The recommendations
for governance of opt-out providers and building variant wheels will
follow in subsequent PEPs.

Signed-off-by: Michał Górny <mgorny@quansight.com>

Author: mgorny