[BitBucket] Replace fstring with format function.

[Spacetown]

@gonchik Can the usage of f-strings be checked with flake8? I found nothing.

Closes #828

[codecov-commenter]

Codecov Report

Merging #830 (392e4f6) into master (e5bff44) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #830   +/-   ##
=======================================
  Coverage   35.73%   35.73%           
=======================================
  Files          32       32           
  Lines        6150     6150           
  Branches      890      890           
=======================================
  Hits         2198     2198           
  Misses       3854     3854           
  Partials       98       98           

Impacted Files	Coverage Δ
atlassian/bitbucket/__init__.py	28.70% <0.00%> (ø)

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[gonchik]

@Spacetown found that one. flake8-no-unnecessary-fstrings .
But I did not find any source code, that's not good to me

[Spacetown]

I also find only checks if a fstring has no code references. Thanks.