have setup.py inspect the python version to determine dependencies

Summary:
Our Arch package is broken because it doesn't install asyncio and
pathlib. These shouldn't *need* to be installed, because Arch is running
Python 3.4, but the executable script that setup.py generates is getting
confused. It seems like the right thing to do is to make sure these
extra dependencies never make it into our `install_requires` list in
Python 3.4. I *think* the fact that our setup.py ends up in the source
distribution means that `pip install peru` will still do the right thing
for Python 3.3.

Test Plan: Tested `pip install .` in a python3.3 virtualenv.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D151

setup.py

@@ -1,5 +1,6 @@
 import os
 import setuptools
+import sys
 
 # Written according to the docs at
 # https://packaging.python.org/en/latest/distributing.html
@@ -25,6 +26,17 @@ def get_all_resources_filepaths():
         resources_paths.extend(relpaths)
     return resources_paths
 
+
+def get_install_requires():
+    dependencies = ['docopt', 'PyYAML']
+    # Python 3.3 needs extra libs that aren't installed by default.
+    if sys.version_info < (3, 3):
+        raise RuntimeError('The minimum supported Python version is 3.3.')
+    elif (3, 3) <= sys.version_info < (3, 4):
+        dependencies.extend(['asyncio', 'pathlib'])
+    return dependencies
+
+
 setuptools.setup(
     name='peru',
     description='A tool for fetching code',
@@ -40,10 +52,5 @@ setuptools.setup(
             'peru=peru.main:main',
         ]
     },
-    install_requires=[
+    install_requires=get_install_requires(),
-        'asyncio',  # Python 3.3.
-        'docopt',
-        'pathlib',  # Python 3.3.
-        'PyYAML',
-    ],
 )