Fix imports and a few logic errors

[dushankw]

1. Imports are broken in Python 3 due to changing of module namespaces:

[$] python -V
Python 3.7.6

[$] python dns-domain-expiration-checker.py
Traceback (most recent call last):
  File "dns-domain-expiration-checker.py", line 24, in <module>
    from email.MIMEMultipart import MIMEMultipart
ModuleNotFoundError: No module named 'email.MIMEMultipart' 

I have added a condition such that the correct names are used for Python 3 while still working in Python 2.

2. There was a type mismatch in the expiry comparison logic, I have fixed it by casting the correct type (str vs binary) on the variable.

3. The output logic for the application only returns a value for days left to expiry if it is < threshold defined by user, I believe this is a mistake and a number should always be output, as currently it will return 0 when a domain is > threshold which implies a valid domain is expired...

[dushankw]

Also add "Expire Date:" to EXPIRE_STRINGS, this is the value used by nic.it

[biji]

good point IMHO, but please rebase to new branch with only fix imports changes