pyproject.toml: Add Python3.6 as available Python version

[corona10]

We dropped the support for Python 3.6 through #142.
Technically, it was available to be installed at Python 3.6 but since pyperformance still uses Python 3.6,
I think that we should sync the version support with the pyperformance team.

There was a restriction at pyproject.toml but it was never published to PyPI yet luckily.

[corona10]

cc @ericsnowcurrently @mdboom

[corona10]

Option A

• Do not maintain Python 3.6 CI
• Downgrade requires-python into Python 3.6

Option B

• Maintain requires-python as Python 3.7
• Drop support Python 3.6 at pyperformance too

Option C

• Maintain Python 3.6 CI
• Downgrade requires-python into Python 3.6

[vstinner]

The CI failed with:

ERROR: Could not find a version that satisfies the requirement setuptools>=61

Is it possible to use older setuptools on Python <= 3.6?

pyproject.toml contains:

[build-system]
requires = ["setuptools >= 61"]
build-backend = "setuptools.build_meta"

Why is version 61 required?

[corona10]

Why is version 61 required?

I think that we can downgrade into 59

[corona10]

@vstinner @ericsnowcurrently @mdboom

There looks some issue between the new package meta system with setuptools and Python 3.6
If we still need to support Python 3.6, I would like to suggest rollback the package build system into setup.py.

[ericsnowcurrently]

After thinking about it, I wouldn't worry about supporting 3.6.

[vstinner]

So pyperformance doesn't use Python 3.7 anymore?

[corona10]

@vstinner we merged python/pyperformance#249, so no support only for 3.6 not 3.7