Fix batching error due to outdated etcd3gw

Testing the new batching feature ended up crashing Neutron:

      File "/usr/local/lib/python3.9/dist-packages/networking_generic_switch/batching.py", line 325, in __init__
        etcd_client = etcd3gw.client(
    TypeError: client() got an unexpected keyword argument 'api_path'

This new feature needs etcd3gw 2.1.0, update minimum version of dependency
accordingly.

Note that this is only an issue when reusing an existing environment or
having to compose with other dependency constraints.  Simply installing
the most recent version of etcd3gw works as expected.

Change-Id: Ie6ecf781fde990ffbce420904978fe5a9fb1e400

Author: jonglezb

requirements.txt

@@ -1,7 +1,7 @@
 # The order of packages is significant, because pip processes them in the order
 # of appearance. Changing the order has an impact on the overall integration
 # process, which may cause wedges in the gate later.
-etcd3gw>=0.2.4 # Apache-2.0
+etcd3gw>=2.1.0 # Apache-2.0
 eventlet>=0.18.2 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 netmiko>=4.1.1 # MIT