Drop `get_package_version_from_pypi` and read current deployed version from `version.txt` file

[mpangrazzi]

Currently, on Hayhooks app instance creation we read the last package version from PyPI.

The purpose is to show on Swagger / Redocly doc pages the current Hayhooks version.

BTW, doing an HTTP call each time the server starts is not ideal, especially because in air-gapped network it could take up to 30s to wait for a (failed) DNS resolution.

We need to:

• Add a version.txt file (or so) with the current version and read it from this file
• Update this file on every new release

cc @anakin87