Reduce build requirements, and ensure they are enforced in testing

[mborland]

Closes: #131

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.83%. Comparing base (7363de5) to head (ee39122).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #132   +/-   ##
========================================
  Coverage    95.83%   95.83%           
========================================
  Files          114      114           
  Lines         8115     8115           
========================================
  Hits          7777     7777           
  Misses         338      338           

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[Lastique]

Note that you should normally not need to add any requirements to tests (or any other downstream users of the library), beyond those which are required by the tests themselves. For that, you would add the requirements that need to be fulfilled by the library users in the usage-requirements of the library. That is, requirements is what library itself needs for compiling, and usage-requirements is what users need to fulfill in order to consume the library. For header-only libraries, those are the same.