Corrected import statement (#2354)

* Corrected import statement

This is needed on Ubuntu 20.04

* Changed shebang line to use the same for all files in bin

bin/cleanmodelica.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Deletes all temporary files created by Dymola, starting
    at the current working directory and recursively searching the 
    subdirectories for files.

bin/validateHTML.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #####################################################################
 # This script validates the html code in the info section
 # of all .mo files in the current directory and in all its

bin/verifyFiles.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #####################################################################
 # This script checks mo and mos files for invalid syntax.
 #