C++20 build failure

[frli8848]

I get this when using C++20:

thread_pool.hpp:13:9: warning: the 'bool' keyword is not allowed in a C++20 concept definition 13 | concept bool Callable = requires(Fn f, Args... args) {

[Tyler-Hardin]

Thanks for bringing this to my attention. Please try the latest commit to see if it resolves the issue.