Fix #640 fconf list of list should not merge

.readthedocs.yml

@@ -4,7 +4,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.9"
+    python: "3.13"
   jobs:
     pre_build:
      - curl https://radia.run | bash -s readthedocs

pykern/fconf.py

@@ -457,7 +457,8 @@ def _list(self, new, base):
             # lists don't have base values
             with self._xpath(f"[{i}]"):
                 x = self._do(e, None)
-            if isinstance(x, list):
+            # if list of lists, just append. If expression returns list, then extend
+            if not isinstance(e, list) and isinstance(x, list):
                 res.extend(x)
             else:
                 res.append(x)

pykern/package_data/projex/dot-readthedocs.yml.jinja

@@ -4,7 +4,7 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.9"
+    python: "3.13"
   jobs:
     pre_build:
      - curl https://radia.run | bash -s readthedocs

tests/fconf_data/1.in/1.yml

@@ -29,6 +29,7 @@ one:
   more_items(inside()):
   list:
     - r0
+    - [a, b, c]
     - red()
   mixed: mymap(host, range(3, 5))
   all_hosts: map(host, ${top.hosts})

tests/fconf_data/1.out/res.json

@@ -56,6 +56,11 @@
         ],
         "list": [
             "r0",
+            [
+                "a",
+                "b",
+                "c"
+            ],
             "r1",
             "r2"
         ],