Update setup.py to require Python 3.6 or higher

- remove support for older Python versions.
- minimal Python requirement for msal 1.28.0 is Python 3.6.

Author: Nico Thomaier

setup.py

@@ -29,17 +29,15 @@
     test_suite="tests",
     license="MIT",
     keywords="git",
+    python_requires=">=3.6",
     classifiers=[
         "Development Status :: 4 - Beta",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: MIT License",
         "Topic :: Internet :: WWW/HTTP",
         "Topic :: Software Development :: Libraries",
         "Programming Language :: Python",
-        "Programming Language :: Python :: 2",
-        "Programming Language :: Python :: 2.7",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",