Add pip as test requirement for PEP 660 editable installs (python#15482)

Author: cdce8p

test-requirements.txt

@@ -5,6 +5,7 @@ black==23.3.0  # must match version in .pre-commit-config.yaml
 filelock>=3.3.0
 isort[colors]==5.12.0; python_version >= "3.8"   # must match version in .pre-commit-config.yaml
 lxml>=4.9.1; (python_version<'3.11' or sys_platform!='win32') and python_version<'3.12'
+pip>=21.3.1
 pre-commit
 pre-commit-hooks==4.4.0
 psutil>=4.0