Modernize the code for Python 3.7

[ulgens]

• Applies pyupgrade --py37-plus
• Removed mock from requirements_dev.txt (replaced by unittest.mock)
• Simplify string formatting with a f-string

[codecov-commenter]

Codecov Report

Merging #178 (2b1da9b) into master (3dbb40b) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #178   +/-   ##
=======================================
  Coverage   93.88%   93.88%           
=======================================
  Files           4        4           
  Lines         474      474           
=======================================
  Hits          445      445           
  Misses         29       29           

Impacted Files	Coverage Δ
tests/test_fields.py	96.33% <100.00%> (ø)
tests/utils.py	83.33% <100.00%> (ø)

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