Remove virtualenv<21 version cap when hatch supports virtualenv 21+

[KedoKudo]

Summary

We had to cap virtualenv<21 in pyproject.toml because virtualenv 21.0.0 removed the propose_interpreters attribute from virtualenv.discovery.builtin, which breaks hatch build.

Error:

Environment `hatch-build` is incompatible: module 'virtualenv.discovery.builtin' has no attribute 'propose_interpreters'

Upstream issue

• Bug: Hatch does not work with virtualenv 21.0.0 pypa/hatch#2193

Action needed

Once hatch releases a version that supports virtualenv>=21, remove the <21 cap from the virtualenv dependency in pyproject.toml:

# Change this:
virtualenv = ">=20.36.1,<21"
# To this:
virtualenv = ">=20.36.1"