Await rendered template if neccesary

[TimoRoth]

Latest release of jupyterhub made render_template return a coroutine: jupyterhub/jupyterhub@be211a4

So it needs to be awaited now, or else self.write throws an error.
To keep backwards compatibility, I first check of the result is indeed awaitable.
Could also just drop this check and always await it, if assuming the use of the latest version of jupyterhub is fine.

[jtpio]

Thanks a lot @TimoRoth for this, looks good!

I wonder if it would be possible to test with multiple versions of JupyterHub on CI. Maybe that would require adding a new entry in the matrix for the jupyterhub version, and possibly a new step in the workflow?

https://github.com/plasmabio/tljh-repo2docker/blob/master/.github/workflows/test.yml

[jtpio]

Thanks!

[jtpio]

Could also just drop this check and always await it, if assuming the use of the latest version of jupyterhub is fine.

We probably want to keep this check for now, since TLJH currently uses 1.2.2:

https://github.com/jupyterhub/the-littlest-jupyterhub/blob/18efee665aebd6c59334685eb9929f823ad649b0/tljh/installer.py#L225