MAINT: Make guess parameter keyword only

This parameter will be added as a new parameter, I think it is more descriptive to have
keyword only parameters for non-essential params.

Author: Kai-Striega

numpy_financial/_financial.py

@@ -695,7 +695,7 @@ def rate(nper, pmt, pv, fv, when='end', guess=None, tol=None, maxiter=100, *, ra
     return rn
 
 
-def irr(values, guess=None, *, tol=1e-12, maxiter=100, raise_exceptions=False):
+def irr(values, *, guess=None, tol=1e-12, maxiter=100, raise_exceptions=False):
     """
     Return the Internal Rate of Return (IRR).