Asynchronous IPC: Initial implementation

This commit adds a mixin which sets up a modal operator which:
* Starts a subprocess
* Polls for messages from that subprocesses

blenderpack.py

@@ -21,10 +21,20 @@ SCHEMA_VERSION = 1
 class BadAddon(Exception):
     pass
 
-def fetch(url):
-    # TODO: do conditional request
-    re = requests.get(url)
-    print(re.json())
+def fetch(url, pipe):
+    pipe[0].close()
+    try:
+        # TODO: do conditional request
+        re = requests.get(url)
+        # print(re.json())
+        pipe[1].send(re.headers)
+    finally:
+        pipe[1].close()
+
+
+
+# class Package():
+#     def __init__(self):
 
 
 def parse_blinfo(source: str) -> dict: