Provide minimal supported Ansible version

[przmv]

Right now Machine Stats for Unix-like Systems requires ansible<2.10 (because 2.10 is not yet supported by Red Hat). However, the minimal required version is not specified. This leads to the following issue:

• OS: Debian 10 (buster)
• Ansible 2.7.7 (installed with sudo apt-get install ansible)
• Machine Stats 1.0.1 (installed with pip3 install machine-stats)

$ machine-stats --help
Traceback (most recent call last):
  File "/home/petr_razumov/.local/bin/machine-stats", line 6, in <module>
    from machine_stats import main
  File "/home/petr_razumov/.local/lib/python3.7/site-packages/machine_stats/__init__.py", line 56, in <module>
    from ansible import context
ImportError: cannot import name 'context' from 'ansible' (/usr/lib/python3/dist-packages/ansible/__init__.py)

ansible.context was introduced in v2.8.0

[pranavmalaviya]

More details/discussion on this issue is being done internally in this doc.