console_to_str: ignore invalid utf-8

[trnila]

When tmux window title contains invalid utf-8 sequence, libtmux throws exception, for example:

UnicodeDecodeError: 'utf-8' codec can't decode byte 0x96 in position 9: invalid start byte

This can be archieved with accidentaly cating binary data to stdout or with

$ printf '\033k;\x96\033\'

This issue seems to be fixed in tmux (tmux/tmux@8149bc3) - title is set only if it is valid utf-8.

Could we fix this issue by ignoring invalid utf-8 sequences directly in function console_to_str?

[codecov-io]

Codecov Report

Merging #70 into master will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   79.71%   79.36%   -0.36%     
==========================================
  Files           8        8              
  Lines         848      848              
==========================================
- Hits          676      673       -3     
- Misses        172      175       +3

Impacted Files	Coverage Δ
libtmux/window.py	82.38% <0%> (-0.63%)	⬇️
libtmux/session.py	80.48% <0%> (-0.61%)	⬇️
libtmux/common.py	85.85% <0%> (-0.51%)	⬇️

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