Amend the collapsing the leading slashes to account for the POSIX special case of two leading slashes.

Thanks to Daniel Colascione for noticing.

pep-0428.txt

@@ -292,19 +292,29 @@ Extraneous path separators and ``"."`` components are eliminated, but not
     PurePosixPath('a/../b')
 
 Multiple leading slashes are treated differently depending on the path
-flavour::
+flavour.  They are always retained on Windows paths (because of the UNC
+notation)::
 
-    >>> PurePosixPath('//some/path')
-    PurePosixPath('/some/path')
     >>> PureNTPath('//some/path')
     PureNTPath('\\\\some\\path\\')
 
+On POSIX, they are collapsed except if there are exactly two leading slashes,
+which is a special case in the POSIX specification on `pathname resolution`_
+(this is also necessary for Cygwin compatibility)::
+
+    >>> PurePosixPath('///some/path')
+    PurePosixPath('/some/path')
+    >>> PurePosixPath('//some/path')
+    PurePosixPath('//some/path')
+
 Calling the constructor without any argument creates a path object pointing
 to the logical "current directory"::
 
     >>> PurePosixPath()
     PurePosixPath('.')
 
+.. _pathname resolution: http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap04.html#tag_04_11
+
 
 Representing
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