Support Core Metadata 2.5 (PEP 794, `Import-Name`, `Import-Namespace`)

[konstin]

Currently, warehouse only supports up to Core Metadata 2.4:

warehouse/warehouse/forklift/metadata.py

Line 26 in b70f937

SUPPORTED_METADATA_VERSIONS = {"1.0", "1.1", "1.2", "2.1", "2.2", "2.3", "2.4"}

PEP 794 bumped the latest version to 2.5, introducing two new fields, Import-Name and Import-Namespace. As of time of writing, PyPI rejects uploads with a Metadata version 2.5.