fix: use 'int.to_bytes' and 'int.from_bytes' for py3 (#904)

diff --git a/google/auth/_helpers.py b/google/auth/_helpers.py
index b239fcd..1b08ab8 100644
--- a/google/auth/_helpers.py
+++ b/google/auth/_helpers.py
@@ -17,6 +17,7 @@
 import base64
 import calendar
 import datetime
+import sys
 
 import six
 from six.moves import urllib
@@ -233,3 +234,12 @@
         Union[str|bytes]: The encoded value
     """
     return base64.urlsafe_b64encode(value).rstrip(b"=")
+
+
+def is_python_3():
+    """Check if the Python interpreter is Python 2 or 3.
+
+    Returns:
+        bool: True if the Python interpreter is Python 3 and False otherwise.
+    """
+    return sys.version_info > (3, 0)