Turn off adaptive step if "step" option is given

[hanjinliu]

This PR changes the default behavior of step and adaptive_step.

I like the idea of using adaptive step by default but it should be turned off if step is explicitly given (also in terms of backward compatibility).
It is not straightforward that the step option is ignored unless "adaptive_step": False is specified at the same time.

@magicgui(x={"step": 0.1})  # ignored
def func(x: float): ...

@magicgui(x={"step": 0.1, "adaptive_step": False})  # OK
def func(x: float): ...

To do this, I corresponded step=None to adaptive step mode, and accordingly, the default value of step is changed from 1 to UNSET and step returns None if the widget is in adaptive step mode.
This change might violate compatibility if one checks the value like widget.step == 1 but it seems very rare.

[codecov]

Codecov Report

Merging #425 (0528bf5) into main (0bfb7bd) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #425      +/-   ##
==========================================
+ Coverage   88.94%   88.99%   +0.05%     
==========================================
  Files          30       30              
  Lines        3926     3935       +9     
==========================================
+ Hits         3492     3502      +10     
+ Misses        434      433       -1     

Impacted Files	Coverage Δ
magicgui/backends/_qtpy/widgets.py	87.00% <100.00%> (+0.11%)	⬆️
magicgui/widgets/_bases/ranged_widget.py	87.93% <100.00%> (+1.01%)	⬆️

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[Czaki]

Could you motivate me to why you add None and _Unset here? One of them should be enough.

[hanjinliu]

That's because min and max are also initialized with UNSET... I guess UNSET is for value only (because it might be set to None intentionally) so should we just change the defaults of min, max and step to None?

[Czaki]

I apologize for missing backward compatibility in my PR.

[tlambert03]

Thank you @hanjinliu