Bug 1504750 [wpt PR 13925] - Update pyflakes to 2.0.0, a=testonly

Automatic update from web-platform-testsUpdate pyflakes to 2.0.0 (#13925)

--

wpt-commits: 5cf3713e05fd6c76ed34fc1037f9e4e44d2296be
wpt-pr: 13925

testing/web-platform/tests/tools/requirements_flake8.txt

@@ -1,4 +1,4 @@
 flake8==3.6.0
 pycodestyle==2.4.0
-pyflakes==1.6.0
+pyflakes==2.0.0
 pep8-naming==0.7.0

testing/web-platform/tests/tools/wptserve/wptserve/server.py

@@ -548,7 +548,7 @@ class Http1WebTestRequestHandler(BaseWebTestRequestHandler):
             self.close_connection = True
             return
 
-        except Exception as e:
+        except Exception:
             err = traceback.format_exc()
             if response:
                 response.set_error(500, err)