Fix Python 3.8 compatibility

Dockerfile

@@ -1,4 +1,5 @@
-FROM python:3.9-bullseye
+FROM ubuntu:20.04
+RUN apt-get update && apt-get install -y python3-pip
 
 RUN pip install pytest requests types-requests pytest-cov mypy twine typing-extensions
 COPY . /opt/aleph-message

aleph_message/exceptions.py

@@ -1,2 +1 @@
-class UnknownHashError(ValueError):
-    ...
+class UnknownHashError(ValueError): ...

aleph_message/models/execution/program.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from typing import Literal, Optional
+from typing import Literal, Optional, List
 
 from pydantic import Field
 
@@ -24,7 +24,7 @@ class CodeContent(HashableModel):
     entrypoint: str
     ref: ItemHash  # Must reference a StoreMessage
     interface: Optional[Interface] = None
-    args: Optional[list[str]] = None
+    args: Optional[List[str]] = None
     use_latest: bool = False
 
     @property

aleph_message/models/execution/volume.py

@@ -16,8 +16,7 @@ class AbstractVolume(HashableModel, ABC):
     mount: Optional[str] = None
 
     @abstractmethod
-    def is_read_only(self):
-        ...
+    def is_read_only(self): ...
 
     class Config:
         extra = Extra.forbid