Add missing exact values for atan() and acot()
Add missing exact values for atan() and acot()

New tan() angles are: pi/5, pi/10, 3*pi/10, 5*pi/12.
(2*pi/5 and pi/12 were already implemented before
this commit.)

The constants table is factored out and moved to
class InverseTrigonometricFunction. It is now shared by
atan() and acot().

The docstring for acot() is updated to mention the used
branch cut. Fixes #13679.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy