ci: pin pluggy 1.6.0 so scancode imports in the license gate

Jenkins DeusData#81 failed in the License gate before building: a fresh CI venv
resolved pluggy 1.7.0, and plugincode 32.0.0 (pulled in by
scancode-toolkit 32.5.0) then raises at import:
  ValueError: did not find any 'pre_scan' hooks in plugincode.pre_scan
Jenkins DeusData#80 passed the day before with pluggy 1.6.0. Pinning pluggy==1.6.0
in requirements-ci.txt restores a working scancode. Checked in a clean
venv: with 1.6.0 'scancode --version' succeeds, and with 1.7.0 it raises
the same ValueError.

Author: ngallodev

requirements-ci.txt

@@ -2,3 +2,7 @@
 # Keep this separate from pkg/pypi/requirements-publish.txt: these packages are
 # CI analyzers, not runtime or registry-publishing dependencies.
 scancode-toolkit==32.5.0
+# pluggy 1.7.0 rejects plugincode 32.0.0's pre_scan hookspec module ("did not
+# find any 'pre_scan' hooks"), which breaks `scancode` at import. Keep the
+# last working release until plugincode supports 1.7.
+pluggy==1.6.0