[MRG] {str(x)} → {x}

[DimitriPapadopoulos]

Casting to str is redundant, it is the default.

From Format Specification Mini-Language:

A general convention is that an empty format specification produces the same result as if you had called str() on the value. A non-empty format specification typically modifies the result.

Reference issue

Tasks

• Unit tests added that reproduce issue or prove feature is working
• Fix or feature added
• Documentation and examples updated (if relevant)
• Unit tests passing and coverage at 100% after adding fix/feature
• Type annotations updated and passing with mypy
• Apps updated and tested (if relevant)

[codecov]

Codecov Report

Merging #867 (022e9ed) into master (ca798a7) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #867   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines         8639      8639           
=========================================
  Hits          8639      8639           

Files	Coverage Δ
pynetdicom/service_class.py	100.00% <ø> (ø)