Cap aiohttp <3.0.0

This is to work around the following issue:

  aiohttp requires Python '>=3.5.3' but the running Python is 3.5.2

Which is breaking builds on Ubuntu Xenial.

Change-Id: I656f4e37a12159b60154cd868a5416c6af3e3139
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
diff --git a/requirements.txt b/requirements.txt
index f24f195..7057c5a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -25,6 +25,6 @@
 cachecontrol
 pyjwt
 iso8601
-aiohttp
+aiohttp<3.0.0
 uvloop;python_version>='3.5'
 psutil