Fix function to get if pyperformance is installed in editable mode (#369)

This is used in compile.py: compile_all installs pyperformance for
every version it compiles. If the main pyperformance is installed
in editable mode, this should be propagated down.
The mechamism to do that was broken because was checking the
presence of setup.py, now long gone.

Author: diegorusso

pyperformance/__init__.py

@@ -20,7 +20,7 @@ def is_installed():
 
 def is_dev():
     parent = os.path.dirname(PKG_ROOT)
-    return os.path.exists(os.path.join(parent, 'setup.py'))
+    return os.path.exists(os.path.join(parent, 'pyproject.toml'))
 
 
 def _is_venv():