python: convert trusted certificates filenames to utf-8

Commit e20cf3d4ecb6 encodes the strings containing the certificates filenames
to "ascii". I realized libopenssl, which is being used by pyOpenSSL,
uses fopen() to open these files. On Linux fopen() supports UTF-8
strings out of the box. Hence we can switch the encoding from 'ascii'
to 'utf-8', making possible to support a wider set of characters.