Fix pyproject.toml license information

Fixes:

```

          ********************************************************************************
          Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

          By 2026-Feb-18, you need to update your project and remove deprecated calls
          or your builds will no longer be supported.

          See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
          ********************************************************************************

```

Author: dweindl

pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = [
-    "setuptools>=62",
+    "setuptools>=77",
     "wheel",
 ]
 build-backend = "setuptools.build_meta"
@@ -24,7 +24,7 @@ dependencies = [
     "antlr4-python3-runtime==4.13.1",
     "pydantic>=2.10",
 ]
-license = {text = "MIT License"}
+license = "MIT"
 authors = [
     {name = "The PEtab developers"},
 ]