Add an optional `[build]` table, only used for the "build" sub-command.
Supported entries.
- `paths = [..]` list of strings.
Explicitly list all paths to include.
Python "wheels" and the manifest are automatically included.
- `paths_exclude_pattern = [..]` list of strings.
A `.gitignore` compatible list of patterns.
By default "paths_exclude_pattern" is set to:
[".*", "__pycache__"] when the `[build]` table entry isn't found.
Only one of these options is supported at once since it makes to sense
to exclude paths when they're manually listed.
