title: Pylint invalid constant name
body: <p>I'm receiving a Pylint error regarding my constant: <code>MIN_SOIL_PARTICLE_DENS</code> (invalid name).
  Any ideas why this constant is wrong? Here's my full function:</p>
  
  <pre><code>def bulk_density(clay, sand, organic_matter):
      MIN_SOIL_PARTICLE_DENS = 2.65
      x1 = (0.078 + 0.278 * sand + 0.034 * clay + 0.022 * organic_matter - 0.018
            * sand * organic_matter - 0.027 * clay * organic_matter - 0.584 * sand
            * clay)
      x2 = -0.107 + 1.636 * x1
      field_capacity = vol_water_content_33_j_kg(clay, sand, organic_matter)#m3/m3
      sat_water_content = 0.043 + field_capacity + x2 - 0.097 * sand
      return (1 - sat_water_content) * MIN_SOIL_PARTICLE_DENS
  </code></pre>