add version file, bump version

[chadawagner]

I'm not sure what the official release process is for this repo, but it's been nearly 2 years since the last release, and 3 months since the latest merge of changes that someone might want to use. Unfortunately it can't just be installed from git because setup.py fails due to a missing version.txt file. This PR just adds that file and removes it from .gitignore so it can be installed without a PyPI release, and bumps the version

[chadawagner]

@WarrierRajeev @ravipetlur I'm now remembering that there was a PEP issue with a version like 0.0.8. I'll push another commit bumping it to 0.1.0 but feel free to edit, it's up to y'all what you want to do for a versioning strategy.

[drsnyder]

Can we get this merged? Adding via poetry add git+https://github.com/verloop/twirpy@main (to get the async client) fails because of the missing version.txt.