Log Message:
py-flit_core: update to 3.11.0.

Support for SPDX license expressions and multiple license files,
as detailed in PEP 639:

    license = "BSD-3-Clause"
    license-files = ["LICENSE"]

For now, only a single license identifier is allowed. More complex
expressions describing multiple licenses & expressions may be
supported in a future version.

The metadata format in produced packages is now version 2.4, to
support the expanded license information.