Replace deprecated basestring with str (Py3)

[MmAaXx500]

Description

In Python3 basestring was removed and replaced with str. So I replaced them.

[fabiencastan]

Thanks for the PR and sorry for the huge latency.
For full compatibility, we can do:

try:
    _ = basestring
except NameError:
    # 'basestring' is undefined, so it must be Python 3
    basestring = (str, bytes)

What do you think?

[MmAaXx500]

Yes, you are right!
I will change it shortly.

[fabiencastan]

We still need to maintain compatibility with python 2.
Could you put the code I proposed at the beginning of the file and continue to use basestring?
This way on python 2 it will find the original basestring and on python 3 basestring will be defined as (str, bytes).

[MmAaXx500]

Sorry, I misunderstood what do you mean.
Of course.

[fabiencastan]

@MmAaXx500 Thanks for making the PR and not only reporting the issue!