Update pyproject.toml min python version

[kosiew]

Describe the bug
pyarrow>=11.0.0,<=12.0.1 depends on Python>=3.7

To Reproduce
Attempt to install pyarrow>=11.0.0 on a Python 3.6 environment.
The dependency resolution fails because pyarrow 11.0.0 and later explicitly require Python 3.7 or higher.

Relevant documentation
PyArrow 11.0.0 on PyPI states that Python >=3.7 is required.

Expected behavior
Automatically restrict the installation to a compatible version of pyarrow for environments running Python <3.7.