Code for Python 3.6

Since we can't assume the user wants to build python on their machine just for this.
- Python 3.6 is the default on RHEL 8
- Remove TOML, which wasn't Standard Library until 3.11
- Fix dict merge on line 315:
  ```
# Before
  self.config = DEFAULT_CONFIG | config

  # After (Python 3.5+)
  self.config = {**DEFAULT_CONFIG, **config}

```