GH-82805: Fix handling of single-dot file extensions in pathlib (#118952)

pathlib now treats "`.`" as a valid file extension (suffix). This brings
it in line with `os.path.splitext()`.

In the (private) pathlib ABCs, we add a new `ParserBase.splitext()` method
that splits a path into a `(root, ext)` pair, like `os.path.splitext()`.
This method is called by `PurePathBase.stem`, `suffix`, etc. In a future
version of pathlib, we might make these base classes public, and so users
will be able to define their own `splitext()` method to control file
extension splitting.

In `pathlib.PurePath` we add optimised `stem`, `suffix` and `suffixes`
properties that don't use `splitext()`, which avoids computing the path
base name twice.

Lib/pathlib/_local.py

@@ -361,6 +361,40 @@ class PurePath(PurePathBase):
         tail[-1] = name
         return self._from_parsed_parts(self.drive, self.root, tail)
 
+    @property
+    def stem(self):
+        """The final path component, minus its last suffix."""
+        name = self.name
+        i = name.rfind('.')
+        if i != -1:
+            stem = name[:i]
+            # Stem must contain at least one non-dot character.
+            if stem.lstrip('.'):
+                return stem
+        return name
+
+    @property
+    def suffix(self):
+        """
+        The final component's last suffix, if any.
+
+        This includes the leading period. For example: '.txt'
+        """
+        name = self.name.lstrip('.')
+        i = name.rfind('.')
+        if i != -1:
+            return name[i:]
+        return ''
+
+    @property
+    def suffixes(self):
+        """
+        A list of the final component's suffixes, if any.
+
+        These include the leading periods. For example: ['.tar', '.gz']
+        """
+        return ['.' + ext for ext in self.name.lstrip('.').split('.')[1:]]
+
     def relative_to(self, other, *, walk_up=False):
         """Return the relative path to another path identified by the passed
         arguments.  If the operation is not possible (because this is not