fileName override fails if pyproject.toml exists

[daishi]

Situation

• You override fileName as documented here: https://github.com/UnitedIncome/serverless-python-requirements#customize-requirements-file-name
• You have a pyproject.toml in your root to configure Black.
• You know nothing about Poetry.

What happens

You end up with an error like

  Error: ENOENT: no such file or directory, open '.../.serverless/requirements.txt'
      at Object.fs.openSync (fs.js:646:18)
      at Object.fs.readFileSync (fs.js:551:33)
      at filterRequirementsFile (/home/daishi/src/p4p_backend/node_modules/serverless-python-requirements/lib/pip.js:355:6)
      at generateRequirementsFile (/home/daishi/src/p4p_backend/node_modules/serverless-python-requirements/lib/pip.js:67:5)
...

Diagnosis

Tracing execution to https://github.com/UnitedIncome/serverless-python-requirements/blob/master/lib/pip.js#L65 shows that unless you explicitly set usePoetry: false, all of the sls commands will fail with the above error.

[zygopleural]

Also seeing this error

[aspin]

+1, but this happens even when I don't customize the requirements file name

[bsamuel-ui]

Duplicate of #400 .