Upgrade flake8 and allow it to specify its dependencies

requirements.txt

@@ -1,28 +1,23 @@
 asn1crypto==0.24.0
-astroid==1.4.4
 autopep8==1.4.3
 bcrypt==3.1.4
 cffi==1.11.5
 colorama==0.3.6
 cryptography==2.3.1
 enum34==1.1.6
-flake8==2.5.4
+flake8==3.7.8
 functools32==3.2.3.post2
 idna==2.7
 ipaddress==1.0.22
 jsonschema==2.5.1
 lazy-object-proxy==1.2.1
-mccabe==0.4.0
 paramiko==2.4.1
-pep8==1.7.1
 psutil==5.4.7
 pyasn1==0.4.4
-pycodestyle==2.4.0
 pycparser==2.18
 pycrypto==2.6.1
-pyflakes==1.0.0
 pylint==1.5.4
 PyNaCl==1.2.1
-six==1.10.0
+six==1.12.0
 sleekxmpp==1.3.3
 wrapt==1.10.10