Remove Python 3.7 .. 3.8 from project classifiers

In `pypropject.toml`, the minimal Python version is set to 3.9.
However, classifiers are:
```
    "Programming Language :: Python :: 3",
    "Programming Language :: Python :: 3.7",
    "Programming Language :: Python :: 3.8",
    "Programming Language :: Python :: 3.9",
    "Programming Language :: Python :: 3.10",
    "Programming Language :: Python :: 3.11",
```

This is inconsistent