Only bump tornado for Python versions >= 3.5

Author: WillSewell

requirements.txt

@@ -16,11 +16,12 @@ PyNaCl==1.3.0
 pyOpenSSL==19.0.0
 requests==2.21.0
 six==1.12.0
-tornado==6.0.2
 urllib3==1.25.1
 aiohttp==3.5.4; python_version >= '3.5'
 async-timeout==3.0.1; python_version >= '3.5'
 attrs==19.1.0; python_version >= '3.5'
 idna-ssl==1.1.0; python_version >= '3.5' and python_version < '3.7'
 multidict==4.5.2; python_version >= '3.5'
+tornado==5.1.1; python_version < '3.5'
+tornado==6.0.2; python_version >= '3.5'
 yarl==1.3.0; python_version >= '3.5'