entrypoint needs to run on py35

repo2docker/buildpacks/repo2docker-entrypoint

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# note: must run on Python >= 3.5
 
 # goals:
 # - load environment variables from a login shell (bash -l)
@@ -35,7 +36,7 @@ def get_login_env():
     try:
         return json.loads(last_line)
     except Exception as e:
-        print(f"Error getting login env: {e}", file=sys.stderr)
+        print("Error getting login env: {e}".format(e=e), file=sys.stderr)
         return {}
 
 
@@ -113,7 +114,11 @@ def main():
 
     def relay_signal(sig, frame):
         """Relay a signal to children"""
-        print(f"Forwarding signal {sig} to {child_pgid}")
+        print(
+            "Forwarding signal {sig} to {child_pgid}".format(
+                sig=sig, child_pgid=child_pgid
+            )
+        )
         os.killpg(child_pgid, sig)
 
     # question: maybe use all valid_signals() except a few, e.g. SIGCHLD?