Use self.log.warning instead of warnings.warn

warnings.warn sidesteps all the json logging bits we do,
and repo2docker will produce non-json output that binderhub
then struggles to consume.

repo2docker/app.py

@@ -267,7 +267,7 @@ class Repo2Docker(Application):
         """
         p = get_platform()
         if p == "linux/arm64":
-            warnings.warn(
+            self.log.warning(
                 "Building for linux/arm64 is experimental. "
                 "To use the recommended platform set --Repo2Docker.platform=linux/amd64. "
                 "To silence this warning set --Repo2Docker.platform=linux/arm64."

repo2docker/buildpacks/conda/__init__.py

@@ -355,7 +355,7 @@ class CondaBuildPack(BaseImage):
     @property
     def py2(self):
         """Am I building a Python 2 kernel environment?"""
-        warnings.warn(
+        self.log.warning(
             "CondaBuildPack.py2 is deprecated in 2023.2. Use CondaBuildPack.separate_kernel_env.",
             DeprecationWarning,
             stacklevel=2,

repo2docker/utils.py

@@ -543,5 +543,4 @@ def get_platform():
         # OSX reports arm64
         return "linux/arm64"
     else:
-        warnings.warn(f"Unexpected platform '{m}', defaulting to linux/amd64")
-        return "linux/amd64"
+        raise ValueError("Unsupported platform {m}")