remove duplicate sphinx dependency

The dependency on sphinx was contained twice in the README and in
addition to `docs/add-requirements.txt` also in `requirements.txt`.

README.md

@@ -60,11 +60,8 @@ dependencies (MIT License, Apache License, PSF License)
 
 Development/testing/documentation/example dependencies (see `requirements.txt`):
 * `jsonschema` and its dependencies (MIT License, Apache License, PSF License)
-* `Sphinx` and its dependencies (multiple licenses)
-* `sphinx-rtd-theme` and its dependencies
-* `sphinx-argparse` (MIT License)
 
-Dependencies for building the documentation:
+Dependencies for building the documentation (see `docs/add-requirements.txt`):
 * `Sphinx` and its dependencies (BSD 2-clause License, MIT License, Apache License)
 * `sphinx-rtd-theme` and its dependencies (MIT License, PSF License)
 * `sphinx-argparse` (MIT License)

requirements.txt

@@ -4,6 +4,3 @@ python-dateutil>=2.8,<3.0
 types-python-dateutil
 pyecma376-2>=0.2.4
 urllib3>=1.26,<2.0
-Sphinx~=3.5.3
-sphinx-rtd-theme~=0.5.1
-sphinx-argparse~=0.2.3