Ocfl-py

* Add preliminary handling of OCFL v1.1 (https://github.com/zimeon/ocfl-py/issues/81)
* Additional validation improvements:
* Checks between version state in different version inventories
* Check to see is extra directories look like version directories
* Fix URI scheme syntax check (https://github.com/zimeon/ocfl-py/issues/75)
* Check extensions directory in storage root (https://github.com/zimeon/ocfl-py/issues/76)
* Use additional fixtures in https://github.com/OCFL/fixtures for tests
* Improve handling of inconsistent state between version inventories errors (E066, https://github.com/zimeon/ocfl-py/issues/85)
* Improve handling of fixity block issues (E056/E111, https://github.com/zimeon/ocfl-py/issues/79, https://github.com/zimeon/ocfl-py/issues/87)
* Running tests on Python 3.6, 3.7, 3.8, 3.9

* Validation improvements:
* Correct handling of missing files mentioned in manifest or fixity
* Catch inconsistent id between versions
* Catch forward slash in contentDirectory
* Add test bad inventory examples in `extra_fixtures/bad-objects`

* Add use of `pylint` in addition to `pycodestyle` and `pydocstyle` (was `pep257`). Numerous minor fixes as a result of errors/warnings reported.
* Use additional fixtures in https://github.com/OCFL/fixtures for tests
* Validation:
* Correct missing root inventory from E034 to E063
* Add tests for digests in prior version manifests

* Add ability for `ocfl-validate.py` to validate a standalone inventory

* Add deeply nested text object (`extra_fixtures/good-objects/ten_level_deep_directories.zip`)
* Fix broken extraction of status of support for validation conditions

* Change to use [PyFilesystem2](https://docs.pyfilesystem.org/en/latest/) for filesystem access which allows use of local filesystem, zip files, and S3. The S3 support is preliminary because it has a number of work-arounds to avoid PyFilesystem2's expectation that there are empty directrory objects
* Renames ocfl.version to ocfl.version_metadata for clarity