Remove virtualenv<21 pin once hatch supports virtualenv>=21

[LalatenduMohanty]

Summary

In #930 / commit d35a8c5, we pinned virtualenv<21 across all CI workflows to work around a breaking change in virtualenv 21.0.0 that removed the propose_interpreters attribute from virtualenv.discovery.builtin, which hatch 1.16.4 depends on.

This pin is a temporary workaround and should be removed once hatch releases a version compatible with virtualenv>=21.

Upstream tracking

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

What needs to happen

1. Wait for the upstream hatch issue to be resolved and a new hatch version released
2. Remove 'virtualenv<21' from all pip install lines in:
   • .github/workflows/check.yaml (5 occurrences)
   • .github/workflows/test.yaml (3 occurrences)
3. Verify CI passes without the pin

Files affected

• .github/workflows/check.yaml
• .github/workflows/test.yaml

Related

• fix(ci): pin virtualenv<21 to fix hatch environment creation failure #930
• Commit d35a8c5