Remove a dead project link, update SymPy link

Author: Tim Bielawa

docsite/source/appendices/related_projects.rst

@@ -71,27 +71,5 @@ In contrast, the bitmath module includes classes representing the full
 spectrum of byte and bit based units, out of the box. No conversion or
 derivation code required of the user.
 
-* `Units Homepage & Docs <http://docs.sympy.org/dev/modules/physics/units.html>`_
+* `Units Homepage & Docs <https://docs.sympy.org/latest/modules/physics/units/index.html>`_
 * Download available through ``pip``, or your distribution's package system
-
-
-Unum
-====
-
-
-    *Unum stands for 'unit-numbers'. It is a Python module that allows
-    to define and manipulate true quantities, i.e. numbers with units
-    such as 60 seconds, [...], 30 dollars etc. The module validates
-    unit consistency in arithmetic expressions; it provides also
-    automatic conversion and output formatting. Unum is designed to be
-    reliable, easy-to-use, customizable and open to any unit
-    definition.*
-
-**Unum**, by Pierre X. Denis, is another extensible library for unit
-manipulation. The module does not appear to have seen any activity in
-quite some time. Looking over the docs gives me the impression that it
-also has a tendency to pollute your namespace with objects like ``M``
-and anything else it pre-defines.
-
-* `Unum Homepage and Docs <http://home.scarlet.be/be052320/Unum_tutorial.html>`_
-* `Unum Source Download <http://home.scarlet.be/be052320/download.html>`_