Add icalendar stubs (#11733)

stubs/icalendar/@tests/stubtest_allowlist.txt

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+# Command line app
+icalendar\.cli
+
+# Tests
+icalendar\.tests(\..*)?
+
+# Methods that use `int` to mean `bool`.
+icalendar\.cal\.Component\.get_inline
+icalendar\.cal\.Component\.set_inline

stubs/icalendar/@tests/test_cases/check_cal.py

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+from icalendar.cal import Component
+
+component = Component()
+component.add("summary", "Test 1")
+component.add("dtstart", "2022-01-01", encode=True)
+component.add("dtend", "2022-01-02", encode=False)
+component.add("location", "Test 3", {})
+component.add("dtstamp", "2022-01-03", parameters={}, encode=True)
+component.add("description", "Test 2", parameters={}, encode=False)  # type: ignore