Don't steal reference from existing Python object (JuliaPy#553)

* Don't steal reference from existing Python object

closes JuliaPy#551

* Refactoring: add pyreturn function

* Fix pystealref! in pyjlwrap_getattr

* Fix pystealref! in pyjlwrap_iternext

Segfault with Python 3.7 (JuliaPy#540)

* Use correct Python version in x64 AppVeyor

* Test with Python 3.7 in AppVeyor

* Implement PyDateTime_CAPI change in bpo-10381

simplify pycall! code to eliminate argument-tuple caching … the logic…

… was just way too complicated and hard to get right, for questionable gain

fix non-reentrancy and lack of gc in recent pycall optimization (Juli…

…aPy#534)

* fix non-reentrancy and lack of gc in recent pycall optimization

* default range conversion uses PyInt

new appveyor script based on appveyor.jl (JuliaPy#524)

whoops

Fix more 0.7 deprecations (JuliaPy#505)

* silence cfunction deprecations

* silence some more deprecations

* define and use constructors rather than convert where possible, but also define convert methods

* pymethod is used in only one place, so inline it so that we can avoid using local vars in at-cfunction

* PyError is no longer mutable

* detect changes to Conda location that may occur in Pkg3

* whoops, be consistent

* separate precompile step to get more Travis info, I hope

* missing Conda check

* don't use at-cfunction interpolation in PyGetSetDef (which can be removed entirely for now, as it is unused)

* rm another unused function

* Revert "define and use constructors rather than convert where possible, but also define convert methods"

This reverts commit 65cb003.

* add constructors for convert methods in 0.7

* rm redundant constructor methods

* defining constructor methods seems to give a huge load time slowdown even in 0.7

* more undef deprecations

* PYTHON=Conda to force conda

* make sure bytes conversions work for codeunits arrays

bugfix in numpy scalars (JuliaPy#481) (JuliaPy#499)

more 0.7 fixes

clarify py eval/exec behavior (closes JuliaPy#434)