Add support for Poetry 1.6

[waketzheng]

1. Add support for Poetry 1.6;
2. Upgrade dev dependencies;
3. Drop support for python<3.8.1 because of flake8 requires.

This version of the plugin supports Poetry

= 1.3, < 1.7.

[JacobHenner]

Thank you for your contribution!

Could you edit your commit(s) to include a DCO signature? I've just added instructions to the project's README: https://github.com/arcesium/poetry-plugin-pypi-mirror#sign-your-work

[waketzheng]

Thank you for your contribution!

Could you edit your commit(s) to include a DCO signature? I've just added instructions to the project's README: https://github.com/arcesium/poetry-plugin-pypi-mirror#sign-your-work

Should I sign the old commit?

[JacobHenner]

Thank you for your contribution!
Could you edit your commit(s) to include a DCO signature? I've just added instructions to the project's README: https://github.com/arcesium/poetry-plugin-pypi-mirror#sign-your-work

Should I sign the old commit?

Yes please. You can rebase your two commits into a single commit, and sign that commit. Then we should be good to go.

[waketzheng]

@JacobHenner done.