Officially deprecate support for python 3.8

Author: jleibs

docs/content/reference/migration/migration-0-20.md

@@ -25,3 +25,8 @@ You can learn more about Rerun's application model and the different servers and
 `re_query::Caches` has been renamed `re_query::QueryCache`, and similarly for `re_query::CacheKey`.
 
 Note that this doesn't affect `re_dataframe`, where this type was already re-exported as `QueryCache`.
+
+## ❗ Deprecations
+
+Support for Python 3.8 is being deprecated. Python 3.8 is past end-of-life. See: https://devguide.python.org/versions/
+In the next release, we will fully drop support and switch to Python 3.9 as the minimum supported version.

rerun_py/rerun_sdk/rerun/__init__.py

@@ -2,6 +2,8 @@
 
 import functools
 import random
+import sys
+import warnings
 from typing import Any, Callable, TypeVar, cast
 from uuid import UUID
 
@@ -10,6 +12,13 @@
 __version__ = "0.20.0-alpha.1+dev"
 __version_info__ = (0, 20, 0, "alpha.1")
 
+
+if sys.version_info < (3, 9):
+    warnings.warn(
+        "Python 3.8 is past EOL (https://devguide.python.org/versions/). Rerun version 0.21 will drop support/testing of Python 3.8.",
+        DeprecationWarning,
+    )
+
 # =====================================
 # API RE-EXPORTS
 # Important: always us the `import _ as _` format to make it explicit to type-checkers that these are public APIs.