Bump Sphinx to 7.0.0

[tompollard]

readthedocs is failing with the following error:

Running Sphinx v4.5.0
loading translations [en]... done
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinx/registry.py", line 438, in load_extension
    metadata = setup(app)
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinxcontrib/applehelp/__init__.py", line 230, in setup
    app.require_sphinx('5.0')
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinx/application.py", line 393, in require_sphinx
    raise VersionRequirementError(version)
sphinx.errors.VersionRequirementError: 5.0
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinx/cmd/build.py", line 272, in build_main
    app = Sphinx(args.sourcedir, args.confdir, args.outputdir,
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinx/application.py", line 219, in __init__
    self.setup_extension(extension)
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinx/application.py", line 380, in setup_extension
    self.registry.load_extension(self, extname)
  File "/home/docs/checkouts/readthedocs.org/user_builds/wfdb/envs/latest/lib/python3.9/site-packages/sphinx/registry.py", line 441, in load_extension
    raise VersionRequirementError(
sphinx.errors.VersionRequirementError: The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.
Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.

This pull request fixes the build by:

• bumping the version of Sphinx to 7.0.0
• fixing the language to "en" in the configuration file

[tompollard]

Build fixed: https://wfdb.readthedocs.io/en/latest/