Draft: Use PEP 517 workflow, Use SPDX license, Depend on python-sqlalchemy

• Depend on python-sqlalchemy
• Change to SPDX license identifier
  • https://github.com/sopel-irc/sopel/blob/v7.1.9/COPYING
• Change to a PEP 517 based workflow

diff from 7.1.9 until sqlalchemy 1.4 was allowed b789e0445009dffcb846d6fceb030d406810cef1...v7.1.9 appears to show only one sqlalchemy related change 00f47d833c10801f128270a11515bbd7f01d2572. Looking at the sqlalchemy migration notes for 1.4 migration_14.html#the-url-object-is-now-immutable without that change URL is passed seven named arguments which although generating a deprecation warning is still supported in 1.4

$ python -c "from sqlalchemy.engine.url import URL;URL(drivername='mysql', username='db_user',password='db_pass', host='db_host', port='10',database='db_name', query={'charset': 'utf8mb4'})"
<string>:1: SADeprecationWarning: Calling URL() directly is deprecated and will be disabled in a future release.  The public constructor for URL is now the URL.create() method.

This would allow dropping python-sqlalchemy1.3 for the drop-python-mock-checkdepends todo list.