Modernize packaging

[cdce8p]

• Move project metadata to pyproject.toml
• Add SPDX license identifier. I believe the correct one here is LGPL-3.0-or-later. The alternative would be LGPL-3.0-only.
  https://spdx.org/licenses/LGPL-3.0-or-later.html
  https://spdx.org/licenses/LGPL-3.0-only.html
• Remove LICENSE.txt from manifest. It's included via license-files already.
• Remove wheel dependency. Setuptools doesn't use it anymore.

Metadata diff

 ...
-Author: Raphael Michel
-Author-email: mail@raphaelmichel.de
+Author-email: Raphael Michel <mail@raphaelmichel.de>
-License: GNU Lesser General Public License v3 (LGPLv3)
+License-Expression: LGPL-3.0-or-later
-Home-page: https://github.com/raphaelm/python-fints
+Project-URL: Homepage, https://github.com/raphaelm/python-fints
-Keywords: hbci banking fints
+Keywords: hbci,banking,fints
 ...
-Classifier: License :: OSI Approved :: GNU Lesser General Public License v3 (LGPLv3)
 ...
+Requires-Python: >=3.9
 ...

[cdce8p]

/CC @raphaelm