Fix: Update license field format in pyproject.toml

(cherry picked from commit 26bc5ea)

Author: PierreRaybaut

pyproject.toml

@@ -9,8 +9,7 @@ name = "PlotPy"
 authors = [{ name = "Codra", email = "[email protected]" }]
 description = "Curve and image plotting tools for Python/Qt applications"
 readme = "README.md"
-license = "BSD-3-Clause"
-license-files = ["LICENSE"]
+license = { file = "LICENSE" }
 classifiers = [
     "Development Status :: 5 - Production/Stable",
     "Intended Audience :: Developers",