Study and adapt Software Package Data Exchange Specification

I found [SPDX](https://spdx.github.io/spdx-spec/v2.3/) as part of migrating to `hatch` when I created a sample project and it had a header like

```python
# SPDX-FileCopyrightText: 2024-present Dev Mukherjee
#
# SPDX-License-Identifier: MIT
```

We should study the relevant directives like `SPDX-License-Identifier` and then add it to each relevant header or the project on the whole.