Cf-units

Adopts the latest cftime (`v1.5`) behaviour, including breaking changes.

* `utime`, `julian_day2date` and `date2julian_day` removed (168).
* `num2date` method now returns a `cftime.datetime` by default, with options to override this (168, 172).
* Improvements to type preservation during conversion (164).

This is a bugfix to restore the `num2date` and `utime` behaviour to that before [cftime v1.1, which saw a change](https://github.com/Unidata/cftime/blob/master/Changelog#L129-L131) to return `cftime.datetime` objects by default (165).

Also drops Python 2 since that is now out of support (157).

The next cf-units release should be a major release, which explicitly adopts and documents cftime's post-1.1 behaviour referenced above.

- This patch releases moves cython from a run requirement to a setup (build) requirement.

This patch release includes the following changes:
* Unpin `numpy<1.17.0` (https://github.com/SciTools/cf-units/pull/154)

This patch release includes the following changes:
* Fix to deal with non-Unit equality (150, 149)
* Fixes to `setup.py`, primarily for correctly setting `__NUMPY_SETUP__` on `__builtins__` (153, 152, 151)

* Added a UDUNITS2 -> [La]TeX converter. ``cf_units.tex.tex(unit_string)``


* In order to solve the above, a formal UDUNITS2 grammar and a private parser API was added (140)
* Support for unicode units (e.g. ``cf_units.Unit('π m²')``) (135, 137)
* Replaced use of private ``_Unit`` factory function (131)
* Improve py38 compatibility (142)
* Better UDUNITS2 exception handling for py3 (136)
* Allow pip install without needing numpy pre-installed (145)