constructor: remove pycosat, add pyyaml requirement

[mbargull]

Removed dependency pycosat since this is not a direct requirement of constructor but one of libconda.
On the other hand pyyaml is a requirement for constructor and has been missing.

[ilanschnell]

pycosat is a only an optional dependency of libconda, and pyyaml is a hard dependency of libconda. Therefore pycosat has to stay a dependency, and it is not necessary to add the pyyaml dependency.

[mbargull]

I disagree on both points:

• pyyaml is a direct dependency for constructor and thus must be added. Say you'd (for some unknown reason) remove the .condarc parsing from libconda and in turn remove the then unnecessary pyyaml dependency. This could in effect break constructor.
• You could argue that pycosat is only an optional dependency because it is only needed if libconda.logic or libconda.resolve (which imports libconda.logic and thus pycosat) are used. But according to libconda's readme, libconda.resolve.Resolve is part of its "main functionality":

The main functionality this package provides is to allow importing the
following:

from libconda.fetch import fetch_index, fetch_pkg
from libconda.resolve import Resolve

Hence it sounds surprising that the main functionality is actually declared as being something optional.

[mbargull]

To be able to remove pycosat as a dependency, #103 would need to be adopted beforehand, of course. Sorry I hadn't referenced #103 as a requirement for this before.