Integrate requirements-stubtest.txt into METADATA.toml (#9778)

stubs/Pillow/@tests/requirements-stubtest.txt

@@ -1 +0,0 @@
-olefile

stubs/Pillow/@tests/stubtest_allowlist.txt

@@ -1,4 +1,4 @@
 # Requires Pyside or PyQt to be installed.
-# Not worth adding to requirements-stubtest.txt
+# Not worth adding to tool.stubtest.stubtest_requirements
 # because it's only used as a base type and it's a long install.
 PIL.ImageQt.ImageQt

stubs/Pillow/METADATA.toml

@@ -1,4 +1,5 @@
 version = "9.4.*"
 
 [tool.stubtest]
+stubtest_requirements = ["olefile"]
 ignore_missing_stub = true