update doc for 0.9.23 release

update doc and license for 0.9.22

update doc for 0.9.21

doc update

Merge branch 'nested_interrupts_returns'

clarify that 0.9.17 was the final version to support Py2.7

add 3.8 to travis config

auto-run codecov only if numpy is installed

some doc improvements

Merge pull request lmfit#58 from kevin-plutoshift/safe-numpy

Safe numpy