Don't use aioapns 2.1

Fixes #293.

aioapns 2.1 made a breaking change: one can no longer pass an event loop directly to APNs.__init__. We could change our code to fix it, but (regrettably) I think it's quicker to avoid using that version. See also #293 (comment)

setup.py

@@ -22,7 +22,7 @@
 from setuptools import find_packages, setup
 
 INSTALL_REQUIRES = [
-    "aioapns>=1.10",
+    "aioapns>=1.10,<2.1",
     "attrs>=19.2.0",
     "cryptography>=2.6.1",
     "idna>=2.8",