Merge pull request SciTools#1867 from greglucas/doc-whatsnew-0.20

DOC: whats new v0.20

Merge pull request SciTools#1769 from greglucas/sdist-work

FIX: Adding build_ext call to setup.py on release

Merge pull request SciTools#1703 from greglucas/whatsnew_0.19

DOC: Adding release notes for v0.19

First release candidate for Cartopy v0.19.0.

Cartopy 0.18.0

Major changes include:

* Support for building against Proj 6.
* Support for Python 3.7 and 3.8.
* Support for Matplotlib 3.2 and 3.3.
* Support for labelling gridlines on all projections.
* Support for labelling gridlines within plot boundaries instead of
  edges.
* Support for labelling of contours.

and many others.

There are several bug fixes as well. See [the
milestone](https://github.com/SciTools/cartopy/milestone/25?closed=1)
for more information.

Cartopy 0.18.0 release candidate 1

Major changes include:

* Support for building against Proj 6.
* Support for Python 3.7 and 3.8.
* Support for Matplotlib 3.2 and 3.3.
* Support for labelling gridlines on all projections.
* Support for labelling gridlines within plot boundaries instead of
  edges.
* Support for labelling of contours.

and many others.

There are several bug fixes as well. See [the
milestone](https://github.com/SciTools/cartopy/milestone/25?closed=1)
for more information.

Cartopy 0.18.0 beta 2

Major changes include:

* Support for building against Proj 6.
* Support for Python 3.7 and 3.8.
* Support for Matplotlib 3.2 and 3.3.
* Support for labelling gridlines on all projections.
* Support for labelling gridlines within plot boundaries instead of
  edges.
* Support for labelling of contours.

and many others.

There are several bug fixes as well. See [the
milestone](https://github.com/SciTools/cartopy/milestone/25?closed=1)
for more information.

Cartopy 0.18.0 beta 1

Major changes include:

* Support for building against Proj 6.
* Support for Python 3.7 and 3.8.
* Support for Matplotlib 3.2 and 3.3.
* Support for labelling gridlines on all projections.
* Support for labelling gridlines within plot boundaries instead of
  edges.
* Support for labelling of contours.

and many others.

There are several bug fixes as well. See [the
milestone](https://github.com/SciTools/cartopy/milestone/25?closed=1)
for more information.

Merge pull request SciTools#1200 from dopplershift/whatsnew-0.17

Update what's new for 0.17

Merge pull request SciTools#1028 from pelson/whats_new_0.16

Prepared the what's new for v0.16.