Set license and license-files key in project metadata to follow PEP639 (#3699)

Adhere to https://peps.python.org/pep-0639 and
specify exact variant of license used (BSD-3-Clause).

* Pin setuptools>=77

Initial support added for PEP 639, see
https://setuptools.pypa.io/en/stable/history.html#v77-0-0

* Remove tool.setuptools.license-files

Fixes `setuptools.errors.InvalidConfigError: 'project.license-files' is defined already.
Remove 'tool.setuptools.license-files'` when running `make package`.

Author: weiji14

pyproject.toml

@@ -1,5 +1,5 @@
 [build-system]
-requires = ["setuptools>=64", "setuptools_scm[toml]>=6.2"]
+requires = ["setuptools>=77", "setuptools_scm[toml]>=6.2"]
 build-backend = "setuptools.build_meta"
 
 [project]
@@ -8,6 +8,8 @@ description = "A Python interface for the Generic Mapping Tools"
 readme = "README.md"
 requires-python = ">=3.11"
 authors = [{name = "The PyGMT Developers", email = "[email protected]"}]
+license = "BSD-3-Clause"
+license-files = ["LICENSE.txt"]
 keywords = [
     "cartography",
     "geodesy",
@@ -31,7 +33,6 @@ classifiers = [
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3.12",
     "Programming Language :: Python :: 3.13",
-    "License :: OSI Approved :: BSD License",
 ]
 dependencies = [
     "numpy>=1.25",
@@ -58,9 +59,6 @@ all = [
 "Changelog" = "https://www.pygmt.org/latest/changes.html"
 "Issue Tracker" = "https://github.com/GenericMappingTools/pygmt/issues"
 
-[tool.setuptools]
-license-files = ["LICENSE.txt"]
-
 [tool.setuptools.packages.find]
 include = ["pygmt*"]
 exclude = ["doc"]