Update -relative import documentation to reflect runtime check.

Doc/Manual/Python.html

@@ -5680,8 +5680,9 @@ class M2(pkg2.mod3.M3): pass
 <p>By default, SWIG would generate <tt>mod2.py</tt> proxy file with
 <tt>import</tt> directive as in point 1. This can be changed with the
 <tt>-relativeimport</tt> command line option. The <tt>-relativeimport</tt> instructs
-SWIG to organize imports as in point 2 (for Python 2.x) or as in point 4 (for
+SWIG to organize imports as in point 2 (for Python < 2.7.0) or as in point 4
-Python 3, that is when the -py3 command line option is enabled). In short, if you have
+for Python 2.7.0 and newer. This is a check done at the time the module is
+imported. In short, if you have
 <tt>mod2.i</tt> and <tt>mod3.i</tt> as above, then without
 <tt>-relativeimport</tt> SWIG will write</p>
 
@@ -5696,21 +5697,15 @@ write</p>
 
 <div class="targetlang">
   <pre>
+import sys
+if sys.version_info >= (2, 7, 0):
+    from . import pkg2
+    import pkg1.pkg2.mod3
+else:
     import pkg2.mod3
 </pre>
 </div>
 
-<p>if <tt>-py3</tt> is not used, or</p>
-
-<div class="targetlang">
-<pre>
-from . import pkg2
-import pkg1.pkg2.mod3
-</pre>
-</div>
-
-<p>when <tt>-py3</tt> is used.</p>
-
 <p>You should avoid using relative imports and use absolute ones whenever
 possible. There are some cases, however, when relative imports may be
 necessary. The first example is, when some (legacy) Python code refers entities