Periodicity

- Replaces the RTD docs theme with pydata
- Fixes more version compatibility issues

- Fixes some compatibility issues with recent updates to dependencies.
- GPs using `celerite2.theano` are now deprecated.
- Python 3.7 has been dropped from the testing workflow and Python 3.10 has been added.

- More API changes, now wrapping `xarray.DataArray` to define `TSeries`, `FSeries` and `TFSeries` objects.
- GPs are updated and now work with `george` and `celerite2` using `emcee` and `celerite2.theano` using `pymc3`.
- Methods and docs were updated to work with the new core objects.
- Python 3.6 has been dropped from the testing workflow and Python 3.9 has been added.
- `CompositeSpectrum` added to `periodicity.timefrequency` module.

Basically the same code as the previous version (v1.0b2), bumping merely due to a slight mistake I made with PyPI.

Big changes in the overall structure of the package, preparing for the release of v1.0.0!

In terms of development,
* testing is now automated with tox,
* other tools (black, isort, coverage) are configured in a single pyproject.toml, and
* requirements.txt was replaced by a Pipfile and a Pipfile.lock for reproducible builds with pipenv

And in terms of the API,
* now everything revolves around `periodicity.core.Timeseries` objects, which include several common utilities,
* methods may return `periodicity.core.Periodogram` objects, which have both `frequency` and `period` attributes, and
* in the near future, this will be extended to also include `Spectrogram` objects for time-frequency methods.

Introduces:
* Wavelet Power Spectrum with `PyWavelets` (`periodicity.spectral.wavelet`)
* Empirical Mode Decomposition and the Hilbert-Huang Transform (`periodicity.spectral.emd` and `periodicity.spectral.hht`)
* Phase Dispersion Minimization (`periodicity.spectral.pdm` and `periodicity.spectral.pdm2`)
* Many other useful functions:
* `periodicity.utils.find_extrema`
* `periodicity.utils.find_zero_crossings`
* `periodicity.utils.get_envelope`
* `periodicity.utils.get_noise`

Also, various minor bugfixes and a first documentation release at https://periodicity.readthedocs.io