[Python] Add missing lexer super class imports.

Although, both lexer and parser grammars can have a custom super
class set, but only the generated parser sources were prepared to
import these classes. The patch makes custom lexer ancestor
classes imported, too.

tool/resources/org/antlr/v4/tool/templates/codegen/Python2/Python2.stg

@@ -749,6 +749,10 @@ import sys
 >>
 
 Lexer(lexer, atn, actionFuncs, sempredFuncs, superClass) ::= <<
+<if(superClass)>
+from .<superClass> import <superClass>
+
+<endif>
 
 <atn>
 

tool/resources/org/antlr/v4/tool/templates/codegen/Python3/Python3.stg

@@ -756,7 +756,13 @@ import sys
 >>
 
 Lexer(lexer, atn, actionFuncs, sempredFuncs, superClass) ::= <<
+<if(superClass)>
+if __name__ is not None and "." in __name__:
+    from .<superClass> import <superClass>
+else:
+    from <superClass> import <superClass>
 
+<endif>
 <atn>
 
 class <lexer.name>(<if(superClass)><superClass><else>Lexer<endif>):