set minimal version to 3.7

setup.py

@@ -53,7 +53,7 @@ setuptools.setup(
     package_dir={'': 'src'},
     include_package_data=True,
     zip_safe=False,
-    python_requires=">=3.8",
+    python_requires=">=3.7",
     install_requires=install_requires,
     entry_points = {'console_scripts': ['icalendar = icalendar.cli:main']},
     extras_require={