piuparts-report: fix pickle.loading str from python2

Nis Martensenrequested to merge
nma/piuparts:old_pickles into develop

piuparts-report may encounter old md5cache files pickled by python2 in text mode. Loading such files may fail if there are non-ascii characters. The existing cache files involved in generating the reports on piuparts.debian.org most likely use utf-8 encoding, so let's use that instead of the default assumption 'ascii'.

Should fix:

Traceback (most recent call last):
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1911, in <module>
    main()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1851, in main
    section.generate_output(output_directory, section_names, problem_list, web_host)
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1660, in generate_output
    self.generate_html()
  File "/srv/piuparts.debian.org/share/piuparts/piuparts-report", line 1563, in generate_html
    self._md5cache['old'] = pickle.load(f)
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 71: ordinal not in range(128)

Merge request reports