download: fix logging level of workers on Windows

A gotcha from my Unix-only design. multiprocessing is simply awful
on Windows -- expensive process creation, no signals (only crappy signal
emulation; I miss SIGTSTP in particular), and TIL, no fork. So logger
level set inside a function simply isn't inherited by worker
processes (because it's not a side effect of import).

Were I to redo the whole thing with Windows in mind, I might choose
threads over processes -- but then we face the problem of no reliable
termination mechanism, and a download thread could hang
indefinitely* (unless we dig low enough to be able to inject
synchronization mechanisms into network code, or find a library that's
flexible enough to do so -- certainly not requests or urllib3) unless we
force a dirty exit with unjoined threads.

Trade-offs, trade-offs.

* Make no mistake: iter_content could hang, too. Actually, maybe my
  iter_content_with_timeout
  https://gist.github.com/zmwangx/0dcbdbe6f67f3540dd73e777ef1b2a89 could
  solve the problem within requests, but god knows whether eventlet
  works on Windows, or whether it works but has other gotchas.

src/caterpillar/download.py

@@ -125,12 +125,17 @@ def download_segment(
     )
 
 
-# download_segment wrapper that takes all arguments as a single tuple,
-# so that we can use it with multiprocessing.pool.Pool.map and company.
-# It also gracefully consumes KeyboardInterrupt.
-def _download_segment_mappable(args: Tuple[str, int, pathlib.Path]) -> bool:
+# download_segment wrapper that takes all arguments as a single tuple
+# (with one additional argument: the logging level, so that it can be
+# set correctly for worker processes -- there's no fork on Windows, so
+# the worker processes do not actually inherit logger level), so that we
+# can use it with multiprocessing.pool.Pool.map and company. It also
+# gracefully consumes KeyboardInterrupt.
+def _download_segment_mappable(args: Tuple[str, int, pathlib.Path, int]) -> bool:
     try:
-        return download_segment(*args)
+        *dl_args, logging_level = args
+        logger.setLevel(logging_level)
+        return download_segment(*dl_args)
     except KeyboardInterrupt:
         url, *_ = args
         logger.debug(f"download of {url} has been interrupted")
@@ -172,9 +177,10 @@ def download_m3u8_segments(
     target_duration = remote_m3u8_obj.target_duration
     local_segments = []
     download_args = []
+    logging_level = logger.getEffectiveLevel()
     for index, segment in enumerate(remote_m3u8_obj.segments):
         url = urllib.parse.urljoin(remote_m3u8_url, segment.uri)
-        download_args.append((url, index, local_m3u8_file.parent))
+        download_args.append((url, index, local_m3u8_file.parent, logging_level))
         local_segments.append((f"{index}.ts", segment.duration))
 
     with open(local_m3u8_file, "w", encoding="utf-8") as fp: