drop python<=3.7 support

According to https://endoflife.date/python python 3.7 has been
EOSed 27 Jun 2023.
Filter all code over `pyupgracde --py38-plus`.

Signed-off-by: Tomasz Kłoczko <kloczek@github.com>

src/icalendar/tests/test_with_doctest.py

@@ -51,7 +51,7 @@ def test_docstring_of_python_file(module_name):
         if filename.lower().endswith(".rst")
     ]
 except FileNotFoundError:
-    raise EnvironmentError("Could not find the documentation - remove the build folder and try again.")
+    raise OSError("Could not find the documentation - remove the build folder and try again.")
 
 @pytest.mark.parametrize("filename", [
     "README.rst",