BUG: not converting scalars properly to M8/m8 on assignment (GH6079)
partial fix of #6079

I don't even bother measuring compile time :)

```
Ran 4657 tests in 2032.428s
```
 this was a PITA

@y-p pls check status of the sparc builds maybe in next day or 2 and let me know
 that said...I did find some actual bugs (which for some reason show up on sparc but not on other platforms)....

I think because dtype coercions are slightly different (mainly datetime related)
 :+1: 
 The buildbot builds once every 5 hours btw, because of the long runtime.
 about 45min compile, 30min to run tests! sheesh
 I'm way too young to give you a lecture about punch cards, but, you know...
 before my time too 

hahha
