Fix mbed device-management command for Python 3.8

Bump the `future` module version to 0.18.0 to fix an ImportError present
for Python 3.8:

ImportError: cannot import name 'splitattr' from 'urllib.request' (/usr/lib/python3.8/urllib/request.py)

Author: fkjagodzinski

requirements.txt

@@ -4,7 +4,7 @@ prettytable==0.7.2
 junit-xml==1.8
 pyyaml==4.2b1
 jsonschema==2.6.0
-future==0.16.0
+future>=0.18.0,<1.0
 six==1.12.0
 mbed-cloud-sdk>=2.0.6,<2.1
 requests>=2.20,<2.21