Log Message:
py-flit_core py-flit: updated to 3.10.1

Version 3.10.1

- The sdist of ``flit_core`` now includes the corresponding tests
  These were missing in 3.10.

Version 3.10

- ``flit publish`` can now use PyPI tokens stored in keyring
  either project tokens with a 'username' like
  :samp:`pypi_token:project:{project_name}` (use the `normalised form of the
  name \ 
<https://packaging.python.org/en/latest/specifications/name-normalization/>`_)
  or user tokens
- The ``--python`` option can now take the path of a virtualenv folder, as an
  alternative to a Python executable
- Flit will work with current development versions of Pythona again
- The ``flit`` command line package now requires Python 3.8 or above
  ``flit_core`` still works with Python 3.6 or above.
- The metadata in packages now has the names of optional dependency groups
  ("extras") normalised, complying with version 2.3 of the metadata \ 
standard
- The ``flit`` command line package now depends on pip
- Fix potential substitution of environment variables into passwords read from
  ``.pypirc`` files
- A warning is now shown when building packages which specify the old
  ``flit.buildapi`` backend, which should be replaced by ``flit_core.buildapi``
  build requirement, to protect against changes in future major versions of Flit.
- Avoid using the deprecated ``datetime.utcfromtimestamp()``
- Flit now has a ``SECURITY.md`` file in the Github repository
- The tests for ``flit_core`` are no longer part of the installed package,
  reducing the size of the wheels