Heroku requires python versions specified in runtime.txt prefixed with python for ex: python-3.7
While Netlify only supports just the version number, for ex: 3.7.
Is there any way to bypass the runtime.txt, such as with netlify.toml file?
Nope, sorry, not possible at present. Can heroku be advised to not use it, perhaps?
We are working on the next generation of our CI environment which will be more configurable in aspects like these, as your request is totally reasonable - but I do not have an ETA for release.
Thanks for the info. If either Netlify or Heroku have an alternate way to specify the Python runtime in a different file other than runtime.txt that would solve it. Also, do Netlify or Heroku have a way to execute a command in the build process before the runtime is set?
Can’t really speak to heroku’s featureset, but I guess their tech support could 
There is also no way to do anything during build before we use that file on Netlify’s side.