fix: move aiohttp to extra as it is currently internal surface (#619)

Fix #618. Removes aiohttp from required dependencies to lessen dependency tree for google-auth.

This will need to be looked at again as more folks use aiohttp and once the surfaces goes to public visibility.

setup.py

@@ -27,9 +27,9 @@
     'rsa>=3.1.4,<5; python_version >= "3.5"',
     "setuptools>=40.3.0",
     "six>=1.9.0",
-    'aiohttp >= 3.6.2, < 4.0.0dev; python_version>="3.6"',
 )
 
+extras = {"aiohttp": "aiohttp >= 3.6.2, < 4.0.0dev; python_version>='3.6'"}
 
 with io.open("README.rst", "r") as fh:
     long_description = fh.read()
@@ -47,6 +47,7 @@
     packages=find_packages(exclude=("tests*", "system_tests*")),
     namespace_packages=("google",),
     install_requires=DEPENDENCIES,
+    extras_require=extras,
     python_requires=">=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*",
     license="Apache 2.0",
     keywords="google auth oauth client",