Nskipgrams

Added
- Added support for Python 3.11.

Removed
- Dropped support for Python 3.7.

Added
- Added support for Python 3.10.

Removed
- Dropped support for Python 3.6.

Added
- Added the new class `Skipgrams` (which `Ngrams` subclasses from now).

Changed
- Changed package name from `ngrams` to `nskipgrams`.
- In various methods of `Ngrams`, the argument name `order` has been changed
into `n`.

Removed
- Removed the `total_count` method of `Ngrams`.
Now that `Skipgrams` is added,
`total_count` would be too confusing and/or not of much use in practice.

Fixed
* Avoided creating empty inner tries unnecessarily in the internal tries.

Added
* Implemented the prefix option and multiple orders in `Ngrams.ngrams_with_counts`.
* Implemented `Ngrams.combine` to combine collections of ngrams.

Initial release