Provide way to do full dependency locking

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

If program is provided as python package it contains description of all dependencies if files like setup.cfg or pyproject.toml (ex for poetry). It will be nice to not double this entry in files

**Describe the solution you'd like**
To avoid double providing information it will be nice to read requirements from `setup.cfg` if present and append it to `requires` entry.

**Describe alternatives you've considered**
Manual manage or customs scripts which will overwrite `pyproject.toml`

**Additional context**
I could try to create such PR (but currently does not understand towncrier checks).