feat: add Python 3.9 support, drop Python 3.5 support (#655)

Closes #654.

noxfile.py

@@ -75,7 +75,7 @@ def blacken(session):
     session.run("black", *BLACK_PATHS)
 
 
-@nox.session(python=["3.6", "3.7", "3.8"])
+@nox.session(python=["3.6", "3.7", "3.8", "3.9"])
 def unit(session):
     session.install(*TEST_DEPENDENCIES)
     session.install(*(ASYNC_DEPENDENCIES))
@@ -90,7 +90,7 @@ def unit(session):
     )
 
 
-@nox.session(python=["2.7", "3.5"])
+@nox.session(python=["2.7"])
 def unit_prev_versions(session):
     session.install(*TEST_DEPENDENCIES)
     session.install(".")

setup.py

@@ -23,8 +23,8 @@
     "pyasn1-modules>=0.2.1",
     # rsa==4.5 is the last version to support 2.7
     # https://github.com/sybrenstuvel/python-rsa/issues/152#issuecomment-643470233
-    'rsa<4.6; python_version < "3.5"',
-    'rsa>=3.1.4,<5; python_version >= "3.5"',
+    'rsa<4.6; python_version < "3.6"',
+    'rsa>=3.1.4,<5; python_version >= "3.6"',
     "setuptools>=40.3.0",
     "six>=1.9.0",
 )
@@ -48,17 +48,17 @@
     namespace_packages=("google",),
     install_requires=DEPENDENCIES,
     extras_require=extras,
-    python_requires=">=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*",
+    python_requires=">=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,!=3.5.*",
     license="Apache 2.0",
     keywords="google auth oauth client",
     classifiers=[
         "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",
+        "Programming Language :: Python :: 3.9",
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: Apache Software License",