build: remove wheel from build-system requirements

[alexander-held]

As pointed out by @henryiii (thanks!), wheel is not required in the list of build-system requirements according to PEP 517. See scikit-hep/pylhe#111 and scikit-hep/pyhf#1788.

* remove superfluous wheel requirement in build-system

[codecov]

Codecov Report

Merging #331 (7416561) into master (961fc7c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #331   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         1878      1878           
  Branches       299       299           
=========================================
  Hits          1878      1878           

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