diff --git a/setup.py b/setup.py
index 343e660..301e996 100644
--- a/setup.py
+++ b/setup.py
@@ -20,16 +20,25 @@
 
 
 DEPENDENCIES = (
-    "cachetools >= 2.0.0, < 5.0",
-    "pyasn1-modules >= 0.2.1",
-    "rsa >= 3.1.4, < 5",
-    "setuptools >= 40.3.0",
+    "cachetools>=2.0.0,<5.0",
+    "pyasn1-modules>=0.2.1",
+    # rsa==4.5 is the last version to support 2.7
+    # https://github.com/sybrenstuvel/python-rsa/issues/152#issuecomment-643470233
+    'rsa<4.6; python_version < "3.6"',
+    'rsa>=3.1.4,<5; python_version >= "3.6"',
+    # install enum34 to support 2.7. enum34 only works up to python version 3.3.
+    'enum34>=1.1.10; python_version < "3.4"',
+    "setuptools>=40.3.0",
+    "six>=1.9.0",
 )
 
 extras = {
-    "aiohttp": ["aiohttp >= 3.6.2, < 4.0.0dev", "requests >= 2.20.0, < 3.0.0dev"],
-    "pyopenssl": "pyopenssl >= 20.0.0",
-    "reauth": "pyu2f >= 0.1.5",
+    "aiohttp": [
+        "aiohttp >= 3.6.2, < 4.0.0dev; python_version>='3.6'",
+        "requests >= 2.20.0, < 3.0.0dev",
+    ],
+    "pyopenssl": "pyopenssl>=20.0.0",
+    "reauth": "pyu2f>=0.1.5",
 }
 
 with io.open("README.rst", "r") as fh:
@@ -54,7 +63,7 @@
     namespace_packages=("google",),
     install_requires=DEPENDENCIES,
     extras_require=extras,
-    python_requires=">= 3.6",
+    python_requires=">=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,!=3.5.*",
     license="Apache 2.0",
     keywords="google auth oauth client",
     classifiers=[