Use ==/!= to compare str, bytes, and int literals

flake8 testing of https://github.com/jupyter/kernel_gateway on Python 3.8.0

$ __flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics__
```
./kernel_gateway/notebook_http/request_utils.py:68:18: F632 use ==/!= to compare str, bytes, and int literals
    body = '' if body is b'' or body is None else body.decode(encoding='UTF-8')
                 ^
1     F632 use ==/!= to compare str, bytes, and int literals
1
```

Author: cclauss

kernel_gateway/notebook_http/request_utils.py

@@ -65,7 +65,7 @@ def parse_body(request):
     """
     content_type = TEXT_PLAIN
     body = request.body
-    body = '' if body is b'' or body is None else body.decode(encoding='UTF-8')
+    body = body.decode(encoding='UTF-8') if body else ''
     if 'Content-Type' in request.headers:
         content_type = request.headers['Content-Type']
     return_body = body