use raw strings in docstrings that contain backslashes

For sequences such as '\d' and '\s' that are *not* escape
sequences, this triggers a deprecation warning in recent
versions of Python.

kwant/kpm.py

@@ -21,7 +21,7 @@ __all__ = ['SpectralDensity']
 
 
 class SpectralDensity:
-    """Calculate the spectral density of an operator.
+    r"""Calculate the spectral density of an operator.
 
     This class makes use of the kernel polynomial
     method (KPM), presented in [1]_, to obtain the spectral density

kwant/physics/noise.py

@@ -13,7 +13,7 @@ __all__ = ['two_terminal_shotnoise']
 
 
 def two_terminal_shotnoise(smatrix):
-    """Compute the shot-noise in a two-terminal setup.
+    r"""Compute the shot-noise in a two-terminal setup.
 
     In a two terminal system the shot noise is given by `tr((1 - t*t^\dagger) *
     t*t^\dagger)`.

kwant/solvers/common.py

@@ -555,7 +555,7 @@ class SparseSolver(metaclass=abc.ABCMeta):
 
     def wave_function(self, sys, energy=0, args=(), check_hermiticity=True,
                       *, params=None):
-        """
+        r"""
         Return a callable object for the computation of the wave function
         inside the scattering region.