Fix error formatting
`+` has a lower precedence than the `.format`, so the formatting doesn't actually insert anything. Implicit string concatenation has higher (highest?) precedence.
@@ -359,7 +359,7 @@ class InstallCommand(RequirementCommand):
# so we fail here.
if build_failures:
raise InstallationError(
- "Could not build wheels for {} which use" +
+ "Could not build wheels for {} which use"
" PEP 517 and cannot be installed directly".format(
", ".join(r.name for r in build_failures)))
