require SLiM version 4.0++

Over in MesserLab/SLiM#347 is described a bug leading to memory leaks and possible crashes in the case where reloading from a .trees file while there are Remembered individuals. We would only hit this if there historical samples and a condition on allele frequency. So, this just means we'll want to require SLiM version 4.01 (or whatever the next release that fixes this is).

It could also affect older sims, but as the conditioning code wasn't released, and we're only supporting SLiM v4 moving forward, we needn't worry about it.

This needs to be changed in requirements/ and at https://github.com/popsim-consortium/stdpopsim/blob/main/stdpopsim/slim_engine.py#L1552

There's also a hardcoded check here that could be updated:

stdpopsim/stdpopsim/slim_engine.py

Lines 1551 to 1552 in 3166ead

    
           # SLiM v3.6 sends `stop()` output to stderr, which we rely upon. 
        
           self._assert_min_version("3.6", slim_path)

edit Whoops you already mentioned that!

Fixed by #1567

	# SLiM v3.6 sends `stop()` output to stderr, which we rely upon.
	self._assert_min_version("3.6", slim_path)