Is your feature request related to a problem? Please describe.

Release-please attempts to update a Python package's __init__.py with the new version string.

Some Python projects doesn't use __init__.py to store the version string. Some projects like to place this in a __about__.py file instead. It's right now quite cumbersome to make release-please just update any version string inside the __about__.py file, if it exists.

Describe the solution you'd like

I'd like to have release-please support not only __init__.py but also the __about__.py file. The only difference here is the filename.

Describe alternatives you've considered

I've tried to override the manifest's version-file property, as described here. But it does not have any effect, as this variable is not taken into account in https://github.com/googleapis/release-please/blob/main/src/strategies/python.ts

PR proposal to enable use of version-file: #2434