[Web] Close stale file handle in exec()

Eliminate the Python interpreter warning about failure to close a stale handle when using the `exec(open('%s').read()` idiom, enclosing it in a `with` clause.

src/Mod/Web/Gui/BrowserView.cpp

@@ -544,7 +544,7 @@ void BrowserView::onLinkClicked (const QUrl & url)
 #if PY_MAJOR_VERSION < 3
                     Gui::Command::doCommand(Gui::Command::Gui,"exec(open(unicode('%s', 'utf-8')).read())",(const char*) filename.toUtf8());
 #else
-                    Gui::Command::doCommand(Gui::Command::Gui,"exec(open('%s').read())",(const char*) filename.toUtf8());
+                    Gui::Command::doCommand(Gui::Command::Gui,"with open('%s') as file:\n\texec(file.read())",(const char*) filename.toUtf8());
 #endif
                 }
                 catch (const Base::Exception& e) {