do not import Runner for python 3.5

Author: madhur-ob

metaflow/__init__.py

@@ -147,7 +147,10 @@ class and related decorators.
 from . import tuple_util
 
 # Runner API
-from .metaflow_runner import Runner
+try:
+    from .metaflow_runner import Runner
+except RuntimeError:
+    pass
 
 __version_addl__ = []
 _ext_debug("Loading top-level modules")

metaflow/metaflow_runner.py

@@ -7,6 +7,13 @@
 from metaflow import Run
 from metaflow.subprocess_manager import SubprocessManager, CommandManager
 
+if sys.version_info < (3, 7):
+    raise RuntimeError(
+        """
+        The Metaflow Programmatic API is not supported for versions of Python less than 3.7
+    """
+    )
+
 
 def read_from_file_when_ready(file_path: str, timeout: float = 5):
     start_time = time.time()