ENH: Better handle Praat TextGrids, fix #241

* Be able to parse both "normal" and short format TextGrid files,
  in both UTF-8 and UTF-16 encoding
* Remove empty intervals by default,
  provide option to not remove them
* Be able to load a TextGrid with multiple IntervalTiers
  and then convert to an Annotation with multiple sequences
* Use reprlib with the vendored TextGrid so it is readable

Squash commits:
- Remove _vendor/textgrid
- Move textgrid module into sub-package
- Move test_textgrid into its own subpackage
- Change example TextGrid to a file with more than one tier, for demo purposes
- Add textgrid sub-package in formats/seq/
- Add tests/data_for_tests/textgrid/
- Add test fixtures:
  - a_parse_textgrid_path to fixtures/textgrid.py
  - add textgrid with empty interval fixtures
- Add test_textgrid sub-package in test_formats/test_seq
- Rewrite doc/formats/seq/textgrid.md

doc/formats/seq/textgrid.md

@@ -6,12 +6,7 @@ Annotation format saved by the [Praat](https://www.fon.hum.uva.nl/praat/) applic
 More details about annotating with Praat can be found here:
 <https://www.fon.hum.uva.nl/praat/manual/Intro_7__Annotation.html>
 The specification for TextGrid objects is here: 
-<https://www.fon.hum.uva.nl/praat/manual/TextGrid.html>
-
-Internally, crowsetta uses the Python tool `textgrid`
-(<https://github.com/kylebgorman/textgrid>) to load .TextGrid files. 
-A version is distributed with crowsetta 
-under [MIT license](https://github.com/kylebgorman/textgrid/blob/master/LICENSE).
+<https://www.fon.hum.uva.nl/praat/manual/TextGrid_file_formats.html>
 
 The annotations can be loaded with the following class: 
 {py:class}`crowsetta.formats.seq.textgrid.TextGrid`.