Pattern match artefacts when more than one result

Refactored pattern matching to only match patterns when there's more
than one result. When there's one result, this is assumed to be the
build artefact, which should provide some flexibility for artefact
recognition without sacrificing quality. This also opens the door for
future filtering based on user specified patterns.

sphinx_multiversion/main.py

@@ -428,19 +428,26 @@ def main(argv=None):
                         # example.pdf in the target build directory
                         candidate_files = glob.glob(
                             "{build_dir}/**/*.{extension}".format(
-                                build_dir=target_build_dir, extension=download_format,
+                                build_dir=target_build_dir,
+                                extension=download_format,
                             ),
                             recursive=True,
                         )
-                        build_file_pattern = "{project}.{extension}".format(
-                            project=config.project.replace(" ", ""),
-                            extension=download_format,
-                        )
-                        build_artefacts = [
-                            x
-                            for x in candidate_files
-                            if pathlib.Path(x.lower()).name == build_file_pattern.lower()
-                        ]
+                        if len(candidate_files) > 1:
+                            build_file_pattern = (
+                                "{project}.{extension}".format(
+                                    project=config.project.replace(" ", ""),
+                                    extension=download_format,
+                                )
+                            )
+                            build_artefacts = [
+                                x
+                                for x in candidate_files
+                                if pathlib.Path(x.lower()).name
+                                == build_file_pattern.lower()
+                            ]
+                        else:
+                            build_artefacts = candidate_files
                         if len(build_artefacts) == 0:
                             logger.warning(
                                 (