switch to src layout

src/_pytest/__init__.py

@@ -0,0 +1,8 @@
+__all__ = ["__version__"]
+
+try:
+    from ._version import version as __version__
+except ImportError:
+    # broken installation, we don't even try
+    # unknown only works because we do poor mans version compare
+    __version__ = "unknown"