Fixing pytest-regtest<2.0.0

New releases of `pytest-regtest>=2.0.0` on PyPI are currently broken as the released packages don't appear to include
the Python code for the module itself. This has been reported upstream and when fixed I will remove the dependency.

+ [pytest-regtest #20](https://gitlab.com/uweschmitt/pytest-regtest/-/issues/20)

pyproject.toml

@@ -63,7 +63,7 @@ tests = [
   "pytest-github-actions-annotate-failures",
   "pytest-lazy-fixture",
   "pytest-mpl",
-  "pytest-regtest",
+  "pytest-regtest<2.0.0",
   "filetype",
 ]
 docs = [