Re-enable downstream CI checks (#286)

## Changes

The downstream checks were disabled when we migrated from Hatch, and
this PR removes them entirely: due to compatibility issues with
transitive issues we've needed to specify explicit upper bounds on
dependencies. With this in place downstream projects need to opt-in to
new releases.

### Linked issues

Follows #235.

Author: asnare

.github/workflows/downstreams.yml

@@ -98,7 +98,7 @@ dependencies = [
 
 [dependency-groups]
 test = [
-    "databricks-labs-pytester~=0.7", # <= this library
+    "databricks-labs-pytester~=0.7.5",  # <= this library
     "pytest-cov~=7.0.0",
     "pytest-mock~=3.15.1",
     "pytest-timeout~=2.4.0",