Add #upstream markers for filtered warnings. Add filter for platform module (ref python/cpython#100750).

Author: jaraco

pytest.ini

@@ -2,6 +2,8 @@
 norecursedirs=dist build .tox .eggs
 addopts=--doctest-modules
 filterwarnings=
+	## upstream
+
 	# Ensure ResourceWarnings are emitted
 	default::ResourceWarning
 
@@ -23,3 +25,8 @@ filterwarnings=
 
 	# realpython/pytest-mypy#152
 	ignore:'encoding' argument not specified::pytest_mypy
+
+	# python/cpython#100750
+	ignore::EncodingWarning:platform
+
+	## end upstream