Update app.py

Adding an import statement to the tool for visiting websites (import re - it was causing errors as initially written).

app.py

@@ -155,6 +155,7 @@ def tool_visit_webpage(url:str) -> str:
     Args:
       url: A string url, the site to be visited
     '''
+    import re
     vst = VisitWebpageTool()
     ret = vst(url)
     return ret