add resources symlink

this allows editable installs to find
asdf_standard.resources during an editable
install (via importlib.resources.files)

MANIFEST.in

@@ -0,0 +1 @@
+exclude src/*/resources

src/asdf_standard/integration.py

@@ -1,5 +1,4 @@
 import sys
-from pathlib import Path
 
 if sys.version_info < (3, 9):
     import importlib_resources
@@ -10,13 +9,7 @@
 
 
 def get_resource_mappings():
-    resources_root = importlib_resources.files(asdf_standard) / "resources"
-    if not resources_root.is_dir():
-        # In an editable install, the resources directory will exist off the
-        # repository root:
-        resources_root = Path(__file__).absolute().parent.parent.parent / "resources"
-        if not resources_root.is_dir():
-            raise RuntimeError("Missing resources directory")
+    resources_root = importlib_resources.files("asdf_standard") / "resources"
 
     return [
         asdf_standard.DirectoryResourceMapping(

src/asdf_standard/resources

@@ -0,0 +1 @@
+../../resources