fix: put pytest mock as a dev dependency, not a general one

pyproject.toml

@@ -9,7 +9,6 @@ dependencies = [
   "lxml",  # XML parser
   "nltk",  # For stopwords for the search index
   "platformdirs",  # Get cache dirs and similar by platform
-  "pytest-cov",  # coverage reports for python coverage
   "python-iso639",  # For getting english language names of languages from two letter codes.
   "requests",  # For HTTP requests
   "tdewolff-minify",  # For minification html css and js
@@ -24,6 +23,7 @@ dev = [
   "pillow",  # image processing
   "pyright",  # python typechecker
   "pytest",  # python test runner
+  "pytest-cov",  # coverage reports for python coverage
   "pytest-mock",  # helper for mocking in pytest
   "reuse",  # for enforcing licensing
   "ruff",  # python formatter and linter