title: Signature for ndarray of strings in numba decorator @njit with nopython mode
body: <p>I'm trying to use signature for an array of texts in numba decorator @ njit. For example, this no-sense function:</p>
  
  <pre><code>import numpy as np
  import numba as nb
  @nb.njit(nb.int64
           (nb.types.unicode_type, nb.int8[:], nb.types.unicode_type[:]),
           error_model='numpy')
  def foo(paramText, paramArrayOfInts, paramArrayOfTexts):
      print(paramText)
      return 1
  
  a = 'Hello'
  b = np.array([1, 2], dtype=np.int8)
  c = np.array(['dog', 'cat', 'me'], dtype=np.dtype('U3'))
  aNumber = foo(a, b, c)
  print(aNumber)
  </code></pre>
  
  <p>Shows this error:</p>
  
  <pre><code>  File "C:\OSGeo4W64\apps\Python37\Lib\site-packages\numba\dispatcher.py", line 574, in _explain_matching_error
      raise TypeError(msg) TypeError: No matching definition for argument type(s) unicode_type, array(int8, 1d, C), array([unichr x 3], 1d, C)
  </code></pre>
  
  <p>There is a solution if I insist in using decorators with:</p>
  
  <pre><code>nb.typeof(np.array(['1', '2'], dtype='U3'))
  </code></pre>
  
  <p>instead of:</p>
  
  <pre><code>nb.types.unicode_type[:]
  </code></pre>
  
  <p>It works, but is't not what I'm looking for.</p>
  
  <p>Versions:
  python 3.7
  numpy: 1.18.3
  numba: 0.48.0</p>