This introduces support to Python 3, since Python 2 will
deprecated in January 1st 2020.
More info here: https://www.python.org/doc/sunset-python-2/
This Python 3 conversion breaks compatibility with Python 2
but since Python 2 will be deprecated, I see no issue with that.
Another update this is introducing is ipv6 support since I started
getting my ip in v6 format from the ifconfig providers.
There is a new config entry `subdomains6` for configuring the ipv6
domains that will be updated if the returned ip is ipv6.
With ipv6, the script will update either ipv6 or ipv4 depending on
the value returned by the provider.
The usage of the script remains the same.
Other changes made:
* Use `print()` function instead of the `print` statement
* Better readability by using f-string
* Use the json object directly from `requests`instead of parsing the
content with the `json` module
* Add function to check for ipv6
* Add `subdomains6`` to config got ipv6 subdomains
* Incresed verbosity option
David Ervideira