Remove EmPy 3 compat.

cylc/flow/parsec/empysupport.py

@@ -53,28 +53,18 @@ def empyprocess(
     os.chdir(dir_)
     ftempl = StringIO('\n'.join(flines))
     xtempl = StringIO()
-    # Detect EmPy version by interpreter.file() args.
-    # TODO: Use importlib.metadata version() once we drop Python 3.7
-    if 'name' in inspect.signature(em.Interpreter.file).parameters:
-        # Empy 3
-        interpreter = em.Interpreter(
-            output=em.UncloseableFile(xtempl)
-        )
-    else:
-        # Empy 4
-        # dispatcher = False: raise errors to caller
-        interpreter = em.Interpreter(
-            output=em.UncloseableFile(xtempl),
-            dispatcher=False
-        )
+    # Requires EmPy >= 4.1
+    interpreter = em.Interpreter(
+        output=em.UncloseableFile(xtempl),
+        dispatcher=False  # raise errors to caller
+    )
 
     # Add `CYLC_` environment variables to the global namespace.
     interpreter.updateGlobals(
         get_cylc_env_vars()
     )
 
     try:
-        # These args work for EmPy versions 3 and 4.
         interpreter.file(ftempl, locals=template_vars)
     except Exception as exc:
         lineno = interpreter.contexts[-1].identify()[1]