Restore support for Python 3.5 (#5818)

* Advertise that Python >= 3.6 is needed

ray/tune/examples/ax_example.py contains f-strings which limits support of this package to Python 3.6 and up.

* Python 3.5 does not support f-strings

Rewrite by using format()

* Lower required version after 9f88fe9d

* Remove python_requires again by request

* Fix linter warning

python/ray/tune/examples/ax_example.py

@@ -41,7 +41,7 @@ def easy_objective(config, reporter):
     import time
     time.sleep(0.2)
     for i in range(config["iterations"]):
-        x = np.array([config.get(f"x{i+1}") for i in range(6)])
+        x = np.array([config.get("x{}".format(i + 1)) for i in range(6)])
         reporter(
             timesteps_total=i,
             hartmann6=hartmann6(x),