Add deprecation warning for python 3.9

Change-Id: I2d54e944950deddf8d0946b2b9738bb58e7e955e

Author: BenRKarl

google/ads/googleads/__init__.py

@@ -20,3 +20,11 @@
 import google.ads.googleads.util
 
 VERSION = "27.0.0"
+
+# Checks if the current runtime is Python 3.9.
+if sys.version_info.major == 3 and sys.version_info.minor <= 9:
+    warnings.warn(
+        "Support for Python versions less than 3.9 is deprecated in the "
+        "google-ads package. Please upgrade to Python 3.10 or higher.",
+        category=DeprecationWarning,
+    )