bpo-29455: Mention coverage.py in trace module documentation (GH-435)

(cherry picked from commit 5dfccb06)

Doc/library/trace.rst

@@ -13,6 +13,12 @@ annotated statement coverage listings, print caller/callee relationships and
 list functions executed during a program run.  It can be used in another program
 or from the command line.
 
+.. seealso::
+
+   `Coverage.py <https://coverage.readthedocs.io/>`_
+      A popular third-party coverage tool that provides HTML
+      output along with advanced features such as branch coverage.
+
 .. _trace-cli:
 
 Command-Line Usage