decodetree: Open files with encoding='utf-8'

decodetree: Open files with encoding='utf-8'

When decodetree.py was added in commit 568ae7efae7, QEMU was
using Python 2 which happily reads UTF-8 files in text mode.
Python 3 requires either UTF-8 locale or an explicit encoding
passed to open(). Now that Python 3 is required, explicit
UTF-8 encoding for decodetree source files.

To avoid further problems with the user locale, also explicit
UTF-8 encoding for the generated C files.

Explicit both input/output are plain text by using the 't' mode.

This fixes:

  $ /usr/bin/python3 scripts/decodetree.py test.decode
  Traceback (most recent call last):
    File "scripts/decodetree.py", line 1397, in <module>
      main()
    File "scripts/decodetree.py", line 1308, in main
      parse_file(f, toppat)
    File "scripts/decodetree.py", line 994, in parse_file
      for line in f:
    File "/usr/lib/python3.6/encodings/ascii.py", line 26, in decode
      return codecs.ascii_decode(input, self.errors)[0]
  UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 80:
  ordinal not in range(128)

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Suggested-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210110000240.761122-1-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>