Action fails when `setupotools.setup` values are parsed from text file

[mspronesti]

I'm playing around with a dummy python package to understand how to properly use this action. This is my workflow file:

name: Upload Python Package

on:
  release:
    types: [published]

jobs:
  deploy:

    runs-on: ubuntu-latest

    steps:
    - uses: actions/checkout@v2
    - name: Set up Python
      uses: actions/setup-python@v2
      with:
        python-version: '3.x'
    - name: Install dependencies
      run: |
        python -m pip install --upgrade pip
        pip install build
    - name: Build package
      run: python -m build
    - name: Publish package
      uses: pypa/gh-action-pypi-publish@27b31702a0e7fc50959f5ad993c78deac1bdfc29
      with:
        user: __token__
        password: ${{ secrets.PYPI_API_TOKEN }}

It fails giving

FileNotFoundError: [Errno 2] No such file or directory: '<my_pkg_name>/VERSION.txt'
* Creating venv isolated environment...
* Installing packages in isolated environment... (setuptools >= 40.8.0, wheel)
* Getting dependencies for sdist...
* Building sdist...
* Building wheel from sdist
* Creating venv isolated environment...
* Installing packages in isolated environment... (setuptools >= 40.8.0, wheel)
* Getting dependencies for wheel...

ERROR Backend subproccess exited when trying to invoke get_requires_for_build_wheel
Error: Process completed with exit code 1.

whenever I try to do something like this:

import setuptools
import os

# loading version number from path
VERSION_PATH = os.path.join(os.path.dirname(__file__), "<my_pkg_name>", "VERSION.txt")
with open(VERSION_PATH, "r") as version_file:
    version = version_file.read().strip()

setuptools.setup(
    ...
    version=version,
    ...

If I specify the version number by hand it works as expected.
The very same error appears if I try to parse the description from the README.
Am I doing anything wrong ?