[http-client-python] Use SPDX license metadata for pygen package

## Description

Building `@typespec/http-client-python` with current setuptools emits this warning while building the pygen wheel:

```text
SetuptoolsDeprecationWarning: License classifiers are deprecated.

Please consider removing the following classifiers in favor of a SPDX license expression:

License :: OSI Approved :: MIT License
```

`packages/http-client-python/generator/setup.py` currently specifies both:

```python
license="MIT License"
```

and:

```python
"License :: OSI Approved :: MIT License",
```

Recent setuptools versions expect an SPDX license expression instead of the license classifier.

## Reproduction

From `packages/http-client-python`:

```shell
npm run build
```

The warning appears while setuptools gets build dependencies and again while it builds the wheel.

## Expected behavior

The pygen distribution uses SPDX-compatible license metadata and builds without the setuptools license-classifier deprecation warning.

## Acceptance criteria

- Update the pygen package metadata to use the SPDX expression `MIT`.
- Remove the deprecated `License :: OSI Approved :: MIT License` classifier from pygen's distribution metadata.
- `npm run build` no longer emits `SetuptoolsDeprecationWarning: License classifiers are deprecated` while building the pygen wheel.
- The built wheel continues to report the MIT license correctly in its package metadata.