Use SPDX license expression

[cdce8p]

Use the SPDX license identifier as recommended by PEP 639.
https://spdx.org/licenses/GPL-3.0-or-later.html

homematicip-rest-api/LICENSE.txt

Lines 637 to 640 in 5f26e1d

	This program is free software: you can redistribute it and/or modify
	it under the terms of the GNU General Public License as published by
	the Free Software Foundation, either version 3 of the License, or
	(at your option) any later version.

[hahn-th]

thx! 👍