Add contextvars to pyproject.toml

[rajat404]

Fixes issue raised in #12

P.S: Thanks for building this lib. I always wanted something like this 😍

[mingrammer]

Python 3.7 or later has builtin contextvars library. The contextvars package is only needed for python 3.6.

So, please add the version restriction for contextvars library like this:

contextvars = { version = "^2.4", python = "~3.6" }

Reference: https://python-poetry.org/docs/versions/#python-restricted-dependencies

[rajat404]

Done. Please check 🖖

[mingrammer]

LGTM

I'll release a new version.

[mingrammer]

diagrams v0.2.4 is now released :)