Itertools-len

- [46] Add `batched` (flying-sheep)

- [9] Prettify docs by fixing all strings up (flying-sheep)
- [10] Add tests for everyting – 100% coverage (flying-sheep):

- Fix `len(pairwise([]))`: Now correctly returns `0` instead of `-1`
- Rename `tee.{len→__len__}`
- Fix creating infinite `repeat(x)`: Creation now succeeds, getting `len` fails as expected

- [2] Introduce `pairwise` with a length (bartbroere)
- [3] Fix `dropwhile` being aliased incorrectly (bartbroere)
- Updated CI and bumped minimum Python version to 3.8 as 3.7 ends its maintenance period (flying-sheep)