[ci] Fix specification of nbconvert

Use the `!=` syntax to exclude the unwanted version of `nbconvert` in
requirements.txt since there is no way to specify a logical OR operator in pip
requirements.

requirements.txt

@@ -6,7 +6,7 @@ jupyter
 pytest
 # nbconvert 7.3 has a bug that does not respect --output option
 # See https://github.com/jupyter/nbconvert/issues/1970
-nbconvert < 7.3 , >= 7.4
+nbconvert != 7.3.*
 
 # Needed by tutorials (run as part of roottest)
 pandas