Explicitly set encoding to UTF-8 in postprocess.py

The default (ascii) encoding breaks:

python3.3 postprocess.py html build/html/*.html
Traceback (most recent call last):
  File "postprocess.py", line 59, in <module>
    main()
  File "postprocess.py", line 27, in main
    lines = process_html(fn, f.readlines())
  File "/usr/lib64/python3.3/encodings/ascii.py", line 26, in

    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc2 in
2309: ordinal not in range(128)
Merge pull request #3617 from saschpe/master-postprocess-encoding

Explicitly set encoding to UTF-8 in postprocess.py