simplify dev and base.txt to single files

Instead of having different files for different python versions, just have one,
and use the built in `python_version` to handle which dependencies to install.

requirements/base-py2.txt

@@ -1,4 +0,0 @@
--r base.txt
-
-# Required by Tornado to handle threads stuff.
-futures>=2.0

requirements/base-py3.txt

@@ -1 +0,0 @@
--r base.txt

requirements/base.txt

@@ -5,4 +5,4 @@ MarkupSafe
 requests>=1.0.0
 tornado>=4.2.1,<5.0
 # Required by Tornado to handle threads stuff.
-futures>=2.0
+futures>=2.0; python_version < '3.0'

requirements/dev.txt

@@ -0,0 +1,16 @@
+-r base.txt
+
+mock>=2.0.0
+apache-libcloud>=0.14.0
+boto>=2.32.1
+boto3>=1.2.1
+moto>=0.3.6
+SaltPyLint>=v2017.3.6
+pytest>=3.5.0
+git+https://github.com/saltstack/pytest-salt.git@master#egg=pytest-salt
+
+# httpretty Needs to be here for now even though it's a dependency of boto.
+# A pip install on a fresh system will decide to target httpretty 0.8.10 to
+# satisfy other requirements, and httpretty 0.8.10 has bugs in setup.py that
+# prevent it from being successfully installed (at least on Python 3.4).
+httpretty; python_version >= '3.4'

requirements/dev_python27.txt

@@ -1,11 +1,2 @@
--r base-py2.txt
-
-mock>=2.0.0
-apache-libcloud>=0.14.0
-boto>=2.32.1
-boto3>=1.2.1
-moto>=0.3.6
-SaltPyLint>=v2017.3.6
-pytest>=3.5.0
-git+https://github.com/eisensheng/pytest-catchlog.git@develop#egg=Pytest-catchlog
-git+https://github.com/saltstack/pytest-salt.git@master#egg=pytest-salt
+# This is for legacy purposes
+-r dev.txt

requirements/dev_python34.txt

@@ -1,15 +1,2 @@
--r base-py3.txt
-
-mock>=2.0.0
-apache-libcloud>=0.14.0
-boto>=2.32.1
-boto3>=1.2.1
-moto>=0.3.6
-# httpretty Needs to be here for now even though it's a dependency of boto.
-# A pip install on a fresh system will decide to target httpretty 0.8.10 to
-# satisfy other requirements, and httpretty 0.8.10 has bugs in setup.py that
-# prevent it from being successfully installed (at least on Python 3.4).
-httpretty
-SaltPyLint>=v2017.2.29
-pytest>=3.5.0
-git+https://github.com/saltstack/pytest-salt.git@master#egg=pytest-salt
+# This is a legacy file, use dev.txt
+-r dev.txt