dependencies: pin some dependencies to prevent from constant version changes.

pkgs/python-packages.nix

@@ -674,6 +674,8 b' self: super: {'
     doCheck = true;
     propagatedBuildInputs = [
       self."configobj"
+      self."atomicwrites"
+      self."attrs"
       self."dogpile.cache"
       self."dogpile.core"
       self."decorator"

requirements.txt

@@ -2,6 +2,8 b''
 
 # our custom configobj
 https://code.rhodecode.com/upstream/configobj/archive/a11ff0a0bd4fbda9e3a91267e720f88329efb4a6.tar.gz?md5=9916c524ea11a6c418217af6b28d4b3c#egg=configobj==5.0.6
+atomicwrites==1.1.5
+attrs==18.1.0
 dogpile.cache==0.6.6
 dogpile.core==0.4.1
 decorator==4.1.2