dependencies: pin some dependencies to prevent constant version changes.

pkgs/python-packages.nix

@@ -1653,6 +1653,8 b' self: super: {'
       self."setuptools-scm"
       self."amqp"
       self."authomatic"
+      self."atomicwrites"
+      self."attrs"
       self."babel"
       self."beaker"
       self."celery"

requirements.txt

@@ -4,6 +4,8 b' setuptools-scm==2.1.0'
 amqp==2.3.1
 # not released authomatic that has updated some oauth providers
 https://code.rhodecode.com/upstream/authomatic/archive/90a9ce60cc405ae8a2bf5c3713acd5d78579a04e.tar.gz?md5=3c68720a1322b25254009518d1ff6801#egg=authomatic==0.1.0.post1
+atomicwrites==1.1.5
+attrs==18.1.0
 babel==1.3
 beaker==1.9.1
 celery==4.1.1