Pmdarima

Potentially breaking changes:
* Use of the exogenous keyword (deprecated in 1.8.0) will now raise a `TypeError`
- Use of the `sarimax_kwargs` keyword (deprecated in 1.5.1) will now raise a `TypeError`
- A falsey value for ARIMA's method argument (deprecated pre-1.5.0) will now raise a `ValueError`
- A falsey value for ARIMA's `maxiter` argument will now raise a `ValueError` (warning since 1.5.0)
- `pmdarima` is no longer built for 32-bit architectures
- macOS images are built using macOS 11 instead of macOS 10.15

Other changes:
* Bump numpy dependency to >= 1.21
* Expose `fittedvalues` in the public API. See https://github.com/alkaline-ml/pmdarima/issues/493
* Add support for ARM64 architecture. See https://github.com/alkaline-ml/pmdarima/issues/434
* Introduce new arg, `preserve_series`, to `pmdarima.utils.check_endog` that will preserve or squeeze a Pandas Series object to preserve index information.
* Update Cython pinned version to include `!=0.29.31`

* Add support for Python 3.10
* Remove support for Python 3.6 (EOL: 23 Dec 2021)

* Add compatibility for `statsmodels` 0.13 and higher

* Fix a bug in `tsdisplay` where a value of `lag_max` larger than the length of the series would create a cryptic numpy broadcasting error. This precondition will still cause an error, but now it is one the user can better understand. See 440

* Change `numpy` pin to `numpy>=1.19.3` (and build on lowest supported version) to no longer limit users' NumPy versions. This addresses 449

* Fix a bug where `scikit-learn` version `1.0.0` was raising `ValueError` when calling `if_delegate_has_method`, addressing 454

* Fixes 440
* Test new Circle CI deployment

* Change `numpy` pin to `~=1.19.0` to avoid incompatibility issues, addressing 423