Use git, Change URL, Drop checkdepends, Use PEP 517 workflow, Use SPDX license

• Switch to git source.
  • https://archlinux.org/todo/fix-reproducibility-of-packages-broken-by-pypi-removing-signature-files/
• Update URL
  • Change from launchpad.net mirror to https://github.com/retext-project/pymarkups.
• Drop checkdepends.
  • The PKGBUILD has no check function.
• Change to a PEP 517 based workflow
• Change to SPDX license identifier
  • https://github.com/retext-project/pymarkups/blob/3.1.3/LICENSE