fix(deps): require python 3.6 (#46)

`google-cloud-access-context-manager` is a proto only package that is only used by `google-cloud-asset`. The last version of `google-cloud-asset` that supports python 2.7 is `1.3.0`. See the [setup.py of version 1.3.0 here](https://github.com/googleapis/python-asset/blob/v1.3.0/setup.py#L34). In version `1.3.0` of `python-asset`, the version of `google-cloud-access-context-manager` that is required has an upper bound of `< 0.2.0dev`. Assuming that the SemVer minor version of `google-cloud-access-context-manager` increases after this PR, this change should not break users that are still on `python-asset` version `1.3.0`.

setup.py

@@ -62,18 +62,18 @@
         "License :: OSI Approved :: Apache Software License",
         "Programming Language :: Python",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Operating System :: OS Independent",
         "Topic :: Internet",
     ],
     platforms="Posix; MacOS X; Windows",
     packages=packages,
     namespace_packages=namespaces,
     install_requires=dependencies,
-    python_requires=">=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*",
+    python_requires=">=3.6",
     include_package_data=True,
     zip_safe=False,
 )