refactor!: Update python-spanner dep, drop py 3.5 (#557)

Author: c24t

noxfile.py

@@ -81,7 +81,7 @@ def default(session):
     )
 
 
-@nox.session(python=["3.5", "3.6", "3.7", "3.8"])
+@nox.session(python=["3.6", "3.7", "3.8"])
 def unit(session):
     """Run the unit test suite."""
     default(session)

setup.py

@@ -17,7 +17,7 @@
 # 'Development Status :: 4 - Beta'
 # 'Development Status :: 5 - Production/Stable'
 release_status = "Development Status :: 3 - Alpha"
-dependencies = ["sqlparse >= 0.3.0", "google-cloud-spanner >= 1.8.0"]
+dependencies = ["sqlparse >= 0.3.0", "google-cloud-spanner >= 2.0.0"]
 extras = {}
 
 BASE_DIR = os.path.dirname(__file__)
@@ -53,13 +53,12 @@
         "License :: OSI Approved :: BSD License",
         "Operating System :: OS Independent",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Topic :: Utilities",
         "Framework :: Django",
         "Framework :: Django :: 2.2",
     ],
     extras_require=extras,
-    python_requires=">=3.5",
+    python_requires=">=3.6",
 )