Add v1.x deprecation warning

icepyx/__init__.py

@@ -1,3 +1,17 @@
+from warnings import warn
+
+deprecation_msg = """icepyx v1.x is being deprecated; the back-end systems on which it relies
+will be shut down as of late 2024. At that time, upgrade to icepyx v2.x, which uses the
+new NASA Harmony back-end, will be required. Please see
+<https://icepyx.readthedocs.io/en/latest/user_guide/changelog/v1.3.0.html> for more
+information!
+"""
+# IMPORTANT: This is being done before the other icepyx imports because the imported
+# code changes warning filters. If this is done after the imports, the warning won't
+# work.
+warn(deprecation_msg, FutureWarning, stacklevel=2)
+
+
 from _icepyx_version import version as __version__
 
 from icepyx.core.query import GenQuery, Query