Versioned-hdf5

Major Changes

- Fix issues with the latest version of h5py.

Minor Changes

- Fix an issue where empty datasets deleted with `delete_versions()` could
result in corrupted versions.

- Add info log showing stats about data upon exiting stage_version context.

- Drop support for Python 3.7.

- Remove all Travis CI configuration.

- Various minor cleanups to the test suite.

Minor Changes

- Fix the setting of `prev_version` in `delete_versions()`.

Minor Changes

- Fix variable length strings in _recreate_virtual_dataset. Note that for h5py
3.7.0 they cause a segfault without the upstream fix
[h5py/h5py2111](https://github.com/h5py/h5py/pull/2111).

Minor Changes

- Improve the performance of creating versions in h5py 3.

Minor Changes

- Fix deleting the current version.
- Properly handle fillvalue-only datasets in `delete_versions()`.

Minor Changes

- Fix `delete_versions()` with nested groups.
- Fix `delete_versions()` with variable length strings.