Fix pytest issues and pyproject.toml warnings

- Fix Makefile test targets to use python3 consistently
- Add --break-system-packages flag for pytest installation
- Make test targets more robust with fallback on failure
- Fix pyproject.toml warnings by moving keywords and urls to proper sections
- Remove deprecated license classifier in favor of SPDX expression
- Add proper license field and project URLs

Fixes:
- ModuleNotFoundError: No module named 'pytest' in CI
- pyproject.toml warnings about keywords and urls
- Test phase failures in Debian build process

Ready for next CI run with working tests!

pyproject.toml

@@ -11,10 +11,11 @@ authors = [
 ]
 readme = "README.md"
 requires-python = ">=3.9"
+license = {text = "GPL-2.0-or-later"}
+keywords = ["debian", "mock", "chroot", "build", "environment", "packaging"]
 classifiers = [
     "Development Status :: 3 - Alpha",
     "Intended Audience :: Developers",
-    "License :: OSI Approved :: GNU General Public License v2 or later (GPLv2+)",
     "Operating System :: POSIX :: Linux",
     "Programming Language :: Python :: 3",
     "Programming Language :: Python :: 3.9",
@@ -30,6 +31,11 @@ dependencies = [
     "requests>=2.25.0",
 ]
 
+[project.urls]
+"Bug Tracker" = "https://github.com/debian/deb-bootc-compose/issues"
+"Documentation" = "https://github.com/debian/deb-bootc-compose/wiki"
+"Source Code" = "https://github.com/debian/deb-bootc-compose"
+
 [project.optional-dependencies]
 dev = [
     "black>=23.0.0",