Bump version to 3.4.0

[ogenstad]

It was a while since the last release and at work we are creating a Nornir plugin for our product but due to how the dependencies were setup for mypy-extensions in Nornir 3.3.0 I'm running into an issue when trying to add the latest version of mypy:

❯ poetry add --group dev mypy=1.5.1

Updating dependencies
Resolving dependencies... (0.0s)

Because mypy (1.5.1) depends on mypy-extensions (>=1.0.0)
 and nornir (3.3.0) depends on mypy_extensions (>=0.4.1,<0.5.0), mypy (1.5.1) is incompatible with nornir (3.3.0).
And because no versions of nornir match >3.3.0,<4.0.0, mypy (1.5.1) is incompatible with nornir (>=3.3.0,<4.0.0).

This problem has already been fixed in the main branch, so it would be great if we could release a new version.

Also bumped the Python version in setup.cfg as 3.7 was dropped.

[dbarrosop]

Agreed, a release is long overdue. Any concerns @ktbyers?

[ktbyers]

Sounds good to me.

[ktbyers]

@dbarrosop Let me know if you want me to fix this automated release process:

https://github.com/nornir-automation/nornir/blob/main/.github/workflows/main.yaml#L99

I think I said I would...though I am not sure I want to :-)