chore: update supported python versions in README to >= 3.6 (#668)

* chore: update supported python versions in README to >= 3.6

Update supported python versions in README to >= 3.6, in line with setup.py which indicates python_requires=">=3.6".

* chore: update the deprecated python versions in readme file

Python 3.5 has been deprecated. So, adding it to the list of deprecated versions.

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

README.rst

@@ -56,11 +56,12 @@ dependencies.
 
 Supported Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^
-Python >= 3.5
+Python >= 3.6
 
 Deprecated Python Versions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
-Python == 2.7. Python 2.7 support will be removed on January 1, 2020.
+Python == 2.7.
+Python == 3.5.
 
 
 Mac/Linux