Fix docs to reflect that only Python 3.6.2+ is supported

On Python 3.6.1 nornir import crashes with the following:

Traceback (most recent call last):
  File "test-nornir-01.py", line 3, in <module>
    from nornir.core import InitNornir
  File ".../lib/python3.6/site-packages/nornir/__init__.py", line 1, in <module>
    from nornir.init_nornir import InitNornir
  File ".../lib/python3.6/site-packages/nornir/init_nornir.py", line 4, in <module>
    from nornir.core.connections import Connections
  File ".../lib/python3.6/site-packages/nornir/core/connections.py", line 2, in <module>
    from typing import Any, Dict, NoReturn, Optional, Type
ImportError: cannot import name 'NoReturn'

[ghost]

on python 3.5.2 (default for ubuntu 16.04) nornir is being installed successfully but then crushes on import nornir.core

$ python3
Python 3.5.2 (default, Oct  8 2019, 13:06:37) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import nornir.core
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.5/dist-packages/nornir/__init__.py", line 1, in <module>
    from nornir.init_nornir import InitNornir
  File "/usr/local/lib/python3.5/dist-packages/nornir/init_nornir.py", line 19
    return f"{cls.__module__}.{cls.__name__}"
                                            ^
SyntaxError: invalid syntax

this is because the python version < 3.6 doesn’t support fstrings please add this to your documentation

thx

[ktbyers]

@sindhujit1 Nornir requires Python 3.6.2+.