Enable old Boltzmann key with DeprecationWarning

package/CHANGELOG

@@ -57,6 +57,8 @@ Changes
     (PR #4174, Issue #3819)
 
 Deprecations
+  * Incorrectly spelt Boltzmann unit entry will be removed in version 2.8.0
+    (PR #4230, Issue #4229)
   * The hole2 module is deprecated in favour of the the MDAKit:
     https://github.com/MDAnalysis/hole2-mdakit (Issue #4179, PR #4200)
   * MDAnalysis no longer officially supports 32 bit installations (they are

package/MDAnalysis/units.py

@@ -172,6 +172,23 @@
 
 """
 
+import warnings
+
+
+# Remove in 2.8.0
+class DeprecatedKeyAccessDict(dict):
+    deprecated_kB = 'Boltzman_constant'
+
+    def __getitem__(self, key):
+        if key == self.deprecated_kB:
+            wmsg = ("Please use 'Boltzmann_constant' henceforth. The key "
+                    "'Boltzman_constant' was a typo and will be removed "
+                    "in MDAnalysis 2.8.0.")
+            warnings.warn(wmsg, DeprecationWarning)
+            key = 'Boltzmann_constant'
+        return super().__getitem__(key)
+
+
 #
 # NOTE: Whenever a constant is added to the constants dict, you also
 #       MUST add an appropriate entry to
@@ -189,13 +206,13 @@
 #:    http://physics.nist.gov/Pubs/SP811/appenB8.html#C
 #:
 #: .. versionadded:: 0.9.0
-constants = {
+constants = DeprecatedKeyAccessDict({
     'N_Avogadro': 6.02214129e+23,          # mol**-1
     'elementary_charge': 1.602176565e-19,  # As
     'calorie': 4.184,                      # J
     'Boltzmann_constant': 8.314462159e-3,   # KJ (mol K)**-1
     'electric_constant': 5.526350e-3,      # As (Angstroms Volts)**-1
-}
+})
 
 #: The basic unit of *length* in MDAnalysis is the Angstrom.
 #: Conversion factors between the base unit and other lengthUnits *x* are stored.

testsuite/MDAnalysisTests/utils/test_units.py

@@ -52,13 +52,21 @@ class TestConstants(object):
         ('elementary_charge', 1.602176565e-19),  # As
         ('calorie', 4.184),  # J
         ('Boltzmann_constant', 8.314462159e-3),  # KJ (mol K)**-1
+        ('Boltzman_constant', 8.314462159e-3),  # remove in 2.8.0
         ('electric_constant', 5.526350e-3),  # As (Angstroms Volts)**-1
     )
 
     @pytest.mark.parametrize('name, value', constants_reference)
     def test_constant(self, name, value):
         assert_almost_equal(units.constants[name], value)
 
+    def test_boltzmann_typo_deprecation(self):
+        wmsg = ("Please use 'Boltzmann_constant' henceforth. The key "
+                "'Boltzman_constant' was a typo and will be removed "
+                "in MDAnalysis 2.8.0.")
+        with pytest.warns(DeprecationWarning, match=wmsg):
+            units.constants['Boltzman_constant']
+
 
 class TestConversion(object):
     @staticmethod