tests: fix test-demandimport.py on Python 3.9

Starting with Python 3.9, importing importlib.resources (indirectly) imports
the zipfile module. Therefore, the module is not suitable for the test.
Instead, we can use the ftplib module, which is very unlikely to be imported
during the test run.

tests/test-demandimport.py

@@ -232,7 +232,7 b' from mercurial import util'
 # Unlike the import statement, __import__() function should not raise
 # ImportError even if fromlist has an unknown item
 # (see Python/import.c:import_module_level() and ensure_fromlist())
-assert 'zipfile' not in sys.modules
-zipfileimp = __import__('zipfile', globals(), locals(), ['unknownattr'])
-assert f(zipfileimp) == "<module 'zipfile' from '?'>", f(zipfileimp)
+assert 'ftplib' not in sys.modules
+zipfileimp = __import__('ftplib', globals(), locals(), ['unknownattr'])
+assert f(zipfileimp) == "<module 'ftplib' from '?'>", f(zipfileimp)
 assert not util.safehasattr(zipfileimp, 'unknownattr')