type hints for container.py & handle no stdout

We added the following check as well:

+        if stdout_callback and p.stdout is not None:

Because, according to the subprocess docs[1]:

>  If the stdout argument was not PIPE, this attribute is None.

In this case, it should not need to confirm that p.stdout is not
None in the mypy static analysis. However it still complained. So
we made mypy the favor and confirmed this was the case.

[1]: https://docs.python.org/3/library/subprocess.html#subprocess.Popen.stdout

dangerzone/container.py

@@ -5,6 +5,8 @@ import platform
 import shutil
 import subprocess
 import tempfile
+from collections.abc import Callable
+from typing import List, Optional
 
 import appdirs
 
@@ -28,7 +30,7 @@ log = logging.getLogger(__name__)
 container_name = "dangerzone.rocks/dangerzone"
 
 
-def exec(args, stdout_callback=None):
+def exec(args: List[str], stdout_callback: Callable[[str], None] = None) -> int:
     args_str = " ".join(pipes.quote(s) for s in args)
     log.info("> " + args_str)
 
@@ -41,7 +43,7 @@ def exec(args, stdout_callback=None):
         universal_newlines=True,
         startupinfo=startupinfo,
     ) as p:
-        if stdout_callback:
+        if stdout_callback and p.stdout is not None:
             for line in p.stdout:
                 stdout_callback(line)
 
@@ -49,7 +51,11 @@ def exec(args, stdout_callback=None):
         return p.returncode
 
 
-def exec_container(command, extra_args=[], stdout_callback=None):
+def exec_container(
+    command: List[str],
+    extra_args: List[str] = [],
+    stdout_callback: Callable[[str], None] = None,
+) -> int:
     if container_tech == "podman":
         container_runtime = shutil.which("podman")
         if container_runtime is None:
@@ -84,7 +90,12 @@ def exec_container(command, extra_args=[], stdout_callback=None):
     return exec(args, stdout_callback)
 
 
-def convert(input_filename, output_filename, ocr_lang, stdout_callback):
+def convert(
+    input_filename: str,
+    output_filename: str,
+    ocr_lang: Optional[str],
+    stdout_callback: Callable[[str], None],
+) -> bool:
     success = False
 
     if ocr_lang: