Update python_requires to 3.8 (#176)

## Problem

Recently removed code related to python 3.7 support because it is EOL,
but forgot to include this change to setup.py where we declare what
version of python is required.

## Solution

Adjust the setup.py configuration.

## Type of Change

- [x] Bug fix (non-breaking change which fixes an issue)

Author: jhamon

setup.py

@@ -40,7 +40,7 @@ def read(fname):
         "grpc": read("requirements-grpc.txt"),
     },
     include_package_data=True,
-    python_requires=">=3.6",
+    python_requires=">=3.8",
     entry_points={
         'console_scripts': ['pinecone=pinecone.cli:main'],
     },