* `strings`: Remove unused `hash32_utf8` function
* `strings`: Make `hash_utf8` and `decode_Utf8Str` private
* `strings`: Reorganize private functions
* 'strings': Raise error when non-string/-int types are passed to functions that don't accept them
* `strings`: Add `items()` method, add type hints, remove unused methods, restrict inputs to specific types, reorganize methods
* `Morphology`: Use `StringStore.items()` to enumerate features when pickling
* `test_stringstore`: Update pre-Python 3 tests
* Update `StringStore` docs
* Fix `get_string_id` imports
* Replace redundant test with tests for type checking
* Rename `_retrieve_interned_str`, remove `.get` default arg
* Add `get_string_id` to `strings.pyi`
Remove `mypy` ignore directives from imports of the above
* `strings.pyi`: Replace functions that consume `Union`-typed params with overloads
* `strings.pyi`: Revert some function signatures
* Update `SYMBOLS_BY_INT` lookups and error codes post-merge
* Revert clobbered change introduced in a previous merge
* Remove unnecessary type hint
* Invert tuple order in `StringStore.items()`
* Add test for `StringStore.items()`
* Revert "`Morphology`: Use `StringStore.items()` to enumerate features when pickling"
This reverts commit 1af9510ceb.
* Rename `keys` and `key_map`
* Add `keys()` and `values()`
* Add comment about the inverted key-value semantics in the API
* Fix type hints
* Implement `keys()`, `values()`, `items()` without generators
* Fix type hints, remove unnecessary boxing
* Update docs
* Simplify `keys/values/items()` impl
* `mypy` fix
* Fix error message, doc fixes
* step_through tests: skip instead of xfail
* test_empty_doc should be fixed with new Thinc version
* remove outdated test (there are other misaligned tests now)
* xfail reason
* fix test according to french exceptions
* clarified some skipped tests
* skip ukranian test instead of xfail
* skip instead of xfail
* skip + reason instead of xfail
* removed obsolete tests referring to removed "set_frozen" functionality
* fix test 999
* remove unused AlignmentError
* remove xfail where possible, skip otherwise
* increment thinc release for empty_doc test
* Auto-format tests with black
* Add flake8 config
* Tidy up and remove unused imports
* Fix redefinitions of test functions
* Replace orths_and_spaces with words and spaces
* Fix compatibility with pytest 4.0
* xfail test for now
Test was previously overwritten by following test due to naming conflict, so failure wasn't reported
* Unfail passing test
* Only use fixture via arguments
Fixes pytest 4.0 compatibility
## Description
Related issues: #2379 (should be fixed by separating model tests)
* **total execution time down from > 300 seconds to under 60 seconds** 🎉
* removed all model-specific tests that could only really be run manually anyway – those will now live in a separate test suite in the [`spacy-models`](https://github.com/explosion/spacy-models) repository and are already integrated into our new model training infrastructure
* changed all relative imports to absolute imports to prepare for moving the test suite from `/spacy/tests` to `/tests` (it'll now always test against the installed version)
* merged old regression tests into collections, e.g. `test_issue1001-1500.py` (about 90% of the regression tests are very short anyways)
* tidied up and rewrote existing tests wherever possible
### Todo
- [ ] move tests to `/tests` and adjust CI commands accordingly
- [x] move model test suite from internal repo to `spacy-models`
- [x] ~~investigate why `pipeline/test_textcat.py` is flakey~~
- [x] review old regression tests (leftover files) and see if they can be merged, simplified or deleted
- [ ] update documentation on how to run tests
### Types of change
enhancement, tests
## Checklist
<!--- Before you submit the PR, go over this checklist and make sure you can
tick off all the boxes. [] -> [x] -->
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [ ] My changes don't require a change to the documentation, or if they do, I've added all required information.
