Bump memcached minimum version

python-memcached==1.56 causes the unit tests to fail under python3.7, so
bump to the latest allowed by upper-constraints.

Change-Id: I22a596afcb6b7477f6753ea9896f7ac025be3a85

lower-constraints.txt

@@ -58,7 +58,7 @@ pyparsing==2.2.0
 pyperclip==1.6.0
 python-dateutil==2.7.0
 python-keystoneclient==3.10.0
-python-memcached==1.56
+python-memcached==1.59
 python-mimeparse==1.6.0
 python-subunit==1.2.0
 pytz==2018.3

test-requirements.txt

@@ -15,7 +15,7 @@ stevedore>=1.20.0 # Apache-2.0
 stestr>=2.0.0  # Apache-2.0
 testresources>=2.0.0 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
-python-memcached>=1.56 # PSF
+python-memcached>=1.59 # PSF
 WebTest>=2.0.27 # MIT
 oslo.messaging>=5.29.0 # Apache-2.0