don't leak file in SeekableArchive..

Minor fix is allowing to use pathlib w/ Archive as well..

This also adds a basic test to make sure these features aren't
broken in the future..

When io.BufferedReader (and others) are init-ed based on a file
descriptor, their name attribute will be an integer. Handle these
objects by explicitly checking for an integer type in guess_format() and
setting sane values (empty strings).

We don't trap an exception because Python 2 and 3 throw different types;
AttributeError and TypeError, respectively. So, this way seemed cleaner.

A trailing comma resulted in Entry.pathname being a tuple instead
of a string when using Entry.from_archive() in Python 2.

This partially reverts 5013165958 ("Fix tests for python 2 and 3"),
which accounted for Entry.pathname as a tuple, instead of addressing why
it had become one.

This also adds whitespace around the assignment operator for Python 2
and 3 to look less like a keyword argument assignment (which appears to
be where the comma originally came from).

Add option to set mode to folder.