Glad fails on Windows with Python 3.9
Running Glad via CMake (Generator Ninja) yields the following crash:

```
Generating with args --out-path C:/Projects/MathAnimation/cmake-build-debug/Animations/vendor/gladsources/glad --api gl:core=4.2 c 
Traceback (most recent call last):
  File "C:\Program Files\Python39\lib\runpy.py", line 197, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Program Files\Python39\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Projects\MathAnimation\Animations\vendor\glad\glad\__main__.py", line 194, in <module>
    main()
  File "C:\Projects\MathAnimation\Animations\vendor\glad\glad\__main__.py", line 128, in main
    generators = find_generators()
  File "C:\Projects\MathAnimation\Animations\vendor\glad\glad\plugin.py", line 42, in find_generators
    for entry_point in entry_points(group=entry_point):
  File "C:\Projects\MathAnimation\Animations\vendor\glad\glad\plugin.py", line 11, in entry_points
    return _entry_points()[group]
KeyError: 'glad.generator'
```

System: Windows 10
Build system: CMake
Generator: Ninja

Glad is invoked via:
```
add_subdirectory(glad/cmake)
glad_add_library(glad STATIC API gl:core=4.2)
```

Used commit: f237a2bfcec0d9b82b90ec9af4af265c40de7183