requirements: Add isort development dependency.

requirements/dev.txt

@@ -19,6 +19,9 @@ coverage==4.4.1
 # fake for LDAP testing
 -e git+https://github.com/zulip/fakeldap.git@37bc199e9589c31f4bb5bae1445ad1cd7f84e1ac#egg=fakeldap==0.5.1.zulip1
 
+# For sorting imports
+isort==4.2.15
+
 # for pep8 linter
 pycodestyle==2.3.1
 

requirements/dev_lock.txt

@@ -70,6 +70,7 @@ imagesize==0.7.1
 incremental==17.5.0       # via twisted
 ipython-genutils==0.2.0   # via traitlets
 ipython==6.2.1
+isort==4.2.15
 jedi==0.10.2              # via ipython
 jinja2==2.9.6
 jmespath==0.9.3           # via boto3, botocore