pyproject: quote python version in pyproject.toml

Problem: mypy complains

 pyproject.toml: [mypy]: python_version: Python 3.6 is not supported
   (must be 3.8 or higher). You may need to put quotes around your
   Python version

Put quotes around the python version.

Author: grondo

pyproject.toml

@@ -10,7 +10,7 @@ skip = [
   "src/bindings/python/flux/resource/__init__.py"]
 
 [tool.mypy]
-python_version = 3.6
+python_version = "3.6"
 files = ["src/cmd/**/*.py", "src/bindings/python/flux", "t/python/*.py"]
 mypy_path = ["src/bindings/python", "t/python/tap", "t/python"]
 allow_redefinition = true