chore: add pytest as a dev dependency (#249)

setup.py

@@ -35,5 +35,5 @@ setuptools.setup(
         "docx2txt",
         "pydantic==1.10.8",
     ],
-    extras_require={"dev": ["black", "ruff", "isort"]},
+    extras_require={"dev": ["black", "ruff", "isort", "pytest"]},
 )