platform module: Fixes interpreter version recognition

the previous, human-oriented approach based on Guido's
unmaterialized thought to continue with a version 4 after 3.9.

cerberus/platform.py

@@ -6,18 +6,15 @@
  raise RuntimeError("Cerberus can't be run with Python's optimization level 2.")
 
 
-PYTHON_VERSION = float(sys.version_info[0]) + float(sys.version_info[1]) / 10
-
-
-if PYTHON_VERSION < 3:
+if sys.version_info < (3,):
  _int_types = (int, long) # noqa: F821
  _str_type = basestring # noqa: F821
 else:
  _int_types = (int,)
  _str_type = str
 
 
-if PYTHON_VERSION < 3.3:
+if sys.version_info < (3, 3):
  from collections import (
  Callable,
  Container,
@@ -42,7 +39,7 @@
  Sized,
  )
 
-if PYTHON_VERSION < 3.8:
+if sys.version_info < (3, 8):
  import importlib_metadata
 else:
  import importlib.metadata as importlib_metadata