Update python-ldap version due to install error with 2.5.2

Not sure why this started happening all of a sudden since we have
been using 2.5.2 for quite a while.  But trying to pip install
python-ldap when building a Docker image keeps giving a
UnicodeDecodeError. Updating to version 3.1.0 seems to resolve it.

requirements.txt

@@ -21,7 +21,7 @@ mock==2.0.0
 mysql-python==1.2.5
 packaging==17.1
 phonenumbers==8.8.11
-python-ldap==2.5.2
+python-ldap==3.1.0
 python-memcached==1.59
 service_identity==17.0.0
 simplejson==3.15.0