Strip trailing slash for relative source paths

This fixes an issue introduced in 45cf793
where using `relative_files=True` and `src` with a trailing slash,
the source inserted as `<sources>` in the XML report would also have
a trailing slash. This also fixes an issue introduced in the same
commit where an empty `<source>` would be inserted as well for cases
where the `src` has a trailing slash.

coverage/xmlreport.py

@@ -69,6 +69,8 @@ def __init__(self, coverage: Coverage) -> None:
                 if os.path.exists(src):
                     if not self.config.relative_files:
                         src = files.canonical_filename(src)
+                    else:
+                        src = src.rstrip(r"\/")
                     self.source_paths.add(src)
         self.packages: Dict[str, PackageData] = {}
         self.xml_out: xml.dom.minidom.Document