lib/tevent: Additionally accept unicode as string param in Py2

lib/tevent: Additionally accept unicode as string param in Py2

With the changes to make samba python code Py2/Py3 compatible there
now are many instances where string content is decoded.
Decoded string variables in Py2 are returned as the unicode type. Many
Py2 c-module functions that take string arguments only check for the
string type. However now it's quite possibe the content formally passed
as a string argument is now passed as unicode after being decoded,
such arguments are rejected and code can fail subtly. This only affects
places where the type is directly checked e.g. via PyStr_Check etc.
arguments that are parsed by ParseTuple* functions generally already
accept both string and unicode (if 's', 'z', 's*' format specifiers
are used)

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Alexander Bokovoy <ab@samba.org>