tests: fix "test-client.py" for early python3 versions

ModuleNotFoundError was only introduced in later python 3 versions.
Use just "ImportError", which is the parent class anyway.

Fixes: f7e484c8ed ('tests: fix "test-client.py" ignoring missing "NM" module')
(cherry picked from commit 9902373c6d)

src/tests/client/test-client.py

@@ -111,8 +111,6 @@ import dbus.service
 import dbus.mainloop.glib
 import io
 
-moduleNotFoundError = ModuleNotFoundError if sys.version_info[0] >= 3 else ImportError
-
 import gi
 
 try:
@@ -132,7 +130,7 @@ else:
 
 try:
     import pexpect
-except moduleNotFoundError:
+except ImportError:
     pexpect = None