Update mattermost.py

[kolya7k]

Fix UnicodeEncodeError for non-Latin characters for mattermost destination (#6715)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 63.37%. Comparing base (af0773c) to head (4e48329).
Report is 34 commits behind head on master.

❗ Current head 4e48329 differs from pull request most recent head 4e8fa07. Consider uploading reports for the commit 4e8fa07 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6738      +/-   ##
==========================================
- Coverage   63.82%   63.37%   -0.45%     
==========================================
  Files         161      162       +1     
  Lines       13060    13170     +110     
  Branches     1803     1819      +16     
==========================================
+ Hits         8335     8347      +12     
- Misses       4425     4532     +107     
+ Partials      300      291       -9     

Files	Coverage Δ
redash/destinations/mattermost.py	32.35% <0.00%> (ø)

... and 21 files with indirect coverage changes

[guidopetri]

@kolya7k thanks for the PR! While you're at it, do you think we should update all destinations to support UTF8?

[kolya7k]

Yes, but i encountered with this problem on MM because i'm using MM and Cyrillic symbols :)
I can later make a PR for every destination

[guidopetri]

Could we do it in this PR since it's a one line change?

[eradman]

Rebasing to see if tests pass