Add Flake8 requirement for Python 2.6 and 3.3

python/test_requirements.txt

@@ -1,8 +1,10 @@
-flake8>=2.0
-mock>=1.0.1
-nose>=1.3.7
-PyHamcrest>=1.8.0
+# Flake8 3.x dropped support of Python 2.6 and 3.3
+flake8     <  3.0.0; python_version == '2.6' or python_version == '3.3'
+flake8     >= 3.0.0; python_version == '2.7' or python_version >  '3.3'
+mock       >= 1.0.1
+nose       >= 1.3.7
+PyHamcrest >= 1.8.0
 # This needs to be kept in sync with submodule checkout in
 # third_party/ycmd/third_party
-future==0.15.2
-codecov>=2.0.5
+future     == 0.15.2
+codecov    >= 2.0.5