Update project repository URL and simplify installation instructions in README.md

- Added a repository URL section in `pyproject.toml` to link to the GitHub repository.
- Updated installation instructions in `README.md` to reflect the change from Gitea to GitHub as the source for package installation, removing references to the private Gitea Package Registry.

pyproject.toml

@@ -20,6 +20,9 @@ dependencies = [
     "ttp>=0.10.0",
 ]
 
+[project.urls]
+Repository = "https://github.com/sttarsky/oxipy"
+
 [tool.setuptools.packages.find]
 where = ["."]
 include = ["oxi*"]