prepare release 7.4.0

rc1 updates

update README/NEWS for 7.4.0beta2

Merge pull request tpaviot#718 from tpaviot/review/prepare-r740beta

Review/prepare r740beta

Added 3.8 to travis.yml and appveyor, to check if py3.8 is supported

Merge pull request tpaviot#481 from tpaviot/review/prepare-0.18.1

Upgraded version number to 0.18.1, updated NEWS, doc and README

Merge pull request tpaviot#433 from tpaviot/review/prepare-0.18

Review/prepare 0.18

Update NEWS, README, and version numbers for the coming 0.17.3 release

Merge pull request tpaviot#384 from tpaviot/review/release-0.17.2

Set Version number to 0.17.2; update NEWS and README.md

Merge pull request tpaviot#365 from tpaviot/review/0.17.1-release

Review/0.17.1 release