Neurone-loader

* Bugfix: Fixes parsing errors due to diversly formatted date strings in the NeurOne protocol files.

* ⚠️ This release drops support for end-of-life python versions, most notably Python 2.7. It also
introduces support for more recent python versions up to 3.9.
* Feature: `Recording`, `Session` and `Phase` objects now expose subject information read from the
session protocol XML files as the `subject_info` property. This information is also exported to
MNE objects.
* Feature: Conversion to MNE now sets the `meas_date` in the MNE `Info` object to the recording
start date from the session protocol XML file.

* Bugfix: A crash could occur when trying to load a phase with no events.

* Bugfix: Reproducible ordering of sessions

⚠️ Session loading previously used `os.listdir` to find sessions. This function does not guarantee
any specific order, which might lead to inconsistencies between systems or even runs on the same
system. Sessions are now ordered chronological, i.e. according to their start time, which is
probably what most users expect.

* Maintenance: Tests were migrated from Travis to GitHub Actions

* Github workflow for automatic deployment to PyPI

* Documentation updates for JOSS review