Subprocess management: use a decorator

Use a decorator instead of a mixin to handle subprocess spawning and
monitoring

bpackage/repository.py

@@ -1,8 +1,8 @@
 #!/usr/bin/env python3
 
 # HACK: seems 'requests' module bundled with blender isn't bundled with 'idna' module. So force system python for now
-# import sys
-# sys.path.insert(0, '/usr/lib/python3.6/site-packages')
+import sys
+sys.path.insert(0, '/usr/lib/python3.6/site-packages')
 
 from pathlib import Path
 import requests
@@ -56,10 +56,15 @@ class Repository:
         req_headers = {}
 
         # Do things this way to avoid adding empty objects/None to the req_headers dict
-        if 'etag' in self._headers:
-            req_headers['If-None-Match'] = self._headers['etag']
-        if 'last-modified' in self._headers:
-            req_headers['If-Modified-Since'] = self._headers['last-modified']
+        if self._headers:
+            try:
+                req_headers['If-None-Match'] = self._headers['etag']
+            except KeyError:
+                pass
+            try:
+                req_headers['If-Modified-Since'] = self._headers['last-modified']
+            except KeyError:
+                pass
 
         #try
         resp = requests.get(self.url, headers=req_headers)