follow-up of py36 drop (#3998)

Author: OriolAbril

README.rst

@@ -111,7 +111,7 @@ Another option is to clone the repository and install PyMC3 using
 Dependencies
 ============
 
-PyMC3 is tested on Python 3.6 and depends on Theano, NumPy,
+PyMC3 is tested on Python 3.7 and depends on Theano, NumPy,
 SciPy, and Pandas (see ``requirements.txt`` for version
 information).
 

setup.py

@@ -80,7 +80,7 @@ def get_version():
         # package_data={'docs': ['*']},
         include_package_data=True,
         classifiers=classifiers,
-        python_requires=">=3.6",
+        python_requires=">=3.7",
         install_requires=install_reqs,
         tests_require=test_reqs,
         test_suite="nose.collector",