cubicweb #3649120 __depends__ should support < [open]

__depends__ in __pkginfo__.py currently supports >= or > but not <= or <. And yet this can be useful when maintaining old versions of a cube.

Example:

# cubicweb-ctl list
CubicWeb 3.17.13 (system mode)
[...]
Available instances (/etc/cubicweb.d):
* localizer (all-in-one)
unknown operator <
unknown operator <
#

prioritynormal
typebug
done in<not specified>
load1.000
load left1.000
closed by<not specified>