PY-31442 As in lexers of other languages, reset the state in the skeleton

This way it's more self-contained, and we no longer require to use PythonLexer instead
everywhere. Thus I reverted previously made changes.

python/gen/com/jetbrains/python/lexer/_PythonLexer.java

@@ -870,6 +870,7 @@ return yylength()-s.length();
     zzAtEOF  = false;
     zzAtBOL = true;
     zzEndRead = end;
+    fStringHelper.reset();
     yybegin(initialState);
   }