Drop Python 3.6 Support (linode#224)

## 📝 Description

The status of Python 3.6 is end of life, and may contains unfixed
security issues and other bugs.
https://devguide.python.org/versions/#unsupported-versions

Other cloud provider's tools such as AWS and Azure dropped Python 3.6
support.

https://github.com/boto/boto3/blob/e7cc8bfffa2d53379214f69959645745829b2ce3/setup.py#L40

https://github.com/Azure/azure-sdk-for-python/blob/f81be8cdf1a52fca51b4786af75492d500e4572d/sdk/compute/azure-mgmt-compute/setup.py#L78

I think it is the time for us to make an similar decision.

Author: zliang-akamai

setup.py

@@ -65,9 +65,11 @@ def get_test_suite():
         # that you indicate whether you support Python 2, Python 3 or both.
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
+        'Programming Language :: Python :: 3.10',
+        'Programming Language :: Python :: 3.11',
     ],
 
     # What does your project relate to?
@@ -78,7 +80,7 @@ def get_test_suite():
     packages=find_packages(exclude=['contrib', 'docs', 'test', 'test.*']),
 
     # What do we need for this to run
-    python_requires=">=3.6",
+    python_requires=">=3.7",
 
     install_requires=[
         "requests",