Change license to GPLv3. Use PEP639 license declarations

As discussed and agreed upon with all major contributors, we relax lbmpy's licence to the GPLv3.

Rationale:

[pystencils and lbmpy] are historically distributed under the terms of the Affero GPL v3 (AGPL) license. It has now become clear that this forces every downstream project using either of the two to also adopt the AGPL, instead of the more common General Public License (GPL). Indeed, there are already several GPL projects using our packages, all of which technically constitute a license violation.

We have therefore decided that the best way to move forward is to drop the "A", and change the package licenses to the GPLv3.

[...]

In practice, this changes nothing. The only difference between the two is that the AGPL requires that any service that offers access to the licensed software (or its derivatives) via a computer network must make the source code available. As neither pystencils nor lbmpy qualify as network-interfacing software, this clause is virtually meaningless. Even given the remote chance that downstream projects or forks might involve networking functionality, we are of the opinion that simplifying downstream licensing by using the much more widespread GPL far outweighs this possibility.

This MR furthermore adopts PEP 639 license specification using SPDX identifiers.