scan-build-py: Force the opening in utf-8

It fails on ubuntu bionic otherwise with:
```
scan-build-py-14: Run 'scan-view /tmp/scan-build-2021-08-09-09-14-36-765350-nx9s888s' to examine bug reports.
scan-build-py-14: Internal error.
Traceback (most recent call last):
  File "/usr/lib/llvm-14/lib/libscanbuild/__init__.py", line 125, in wrapper
    return function(*args, **kwargs)
  File "/usr/lib/llvm-14/lib/libscanbuild/analyze.py", line 72, in scan_build
    number_of_bugs = document(args)
  File "/usr/lib/llvm-14/lib/libscanbuild/report.py", line 35, in document
    for bug in read_bugs(args.output, html_reports_available):
  File "/usr/lib/llvm-14/lib/libscanbuild/report.py", line 282, in read_bugs
    for bug in parser(bug_file):
  File "/usr/lib/llvm-14/lib/libscanbuild/report.py", line 421, in parse_bug_html
    for line in handler.readlines():
  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 3360: ordinal not in range(128)
scan-build-py-14: Please run this command again and turn on verbose mode (add '-vvvv' as argument).
```

I guess it is caused by a problem in Python 3.6

Reviewed By: phosek, isthismyaccount

Differential Revision: https://reviews.llvm.org/D107887