Trick rtd into thinking we don't need z3 (#671)

setup.py

@@ -1,5 +1,16 @@
+import os
 from setuptools import setup, find_packages
 
+on_rtd = os.environ.get('READTHEDOCS') == 'True'
+
+
+def rtd_dependent_deps():
+    # RTD tries to build z3, ooms, and fails to build.
+    if on_rtd:
+        return []
+    else:
+        return ['z3-solver']
+
 setup(
     name='manticore',
     description='Manticore is a symbolic execution tool for analysis of binaries and smart contracts.',
@@ -13,8 +24,7 @@ setup(
         'unicorn',
         'ply',
         'pysha3',
-        'z3-solver',
-    ],
+    ] + rtd_dependent_deps(),
     extras_require={
         'dev': [
             'keystone-engine',