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.