Drop unneeded build dependency on coverage

And by doing it, get rid of a tox dependency as well,
as it was only used to get the coverage dependency.

pyparsing.spec

@@ -51,7 +51,8 @@ dos2unix -k examples/*
 
 
 %generate_buildrequires
-%pyproject_buildrequires -t
+# tox lists only the [diagrams] extra and coverage as deps, so we bypass it
+%pyproject_buildrequires -x diagrams
 
 
 %build