Update dev dependencies

Thanks to Github for pointing out a vulnerability in the old requests
version.
The good news is that this is only relevant for developers, but not for
users, as they use the pre-packaged emojis.

requirements-dev.txt

@@ -1,3 +1,3 @@
-beautifulsoup4==4.6.0
-lxml==4.1.0
-requests==2.18.4
+beautifulsoup4==4.6.3
+lxml==4.2.5
+requests==2.20.0