make python 3.6 the minimum required version.

setup.py

@@ -11,8 +11,8 @@
 import versioneer
 
 
-if sys.version_info < (3, 7):
-    print("At least Python 3.7 is required.\n", file=sys.stderr)
+if sys.version_info < (3, 6):
+    print("At least Python 3.6 is required.\n", file=sys.stderr)
     exit(1)
 
 
@@ -91,7 +91,7 @@
         "Intended Audience :: Science/Research",
         "License :: OSI Approved :: MIT License",
         "Natural Language :: English",
-        "Programming Language :: Python :: 3.5",
+        "Programming Language :: Python :: 3.6",
         "Topic :: Scientific/Engineering :: Bio-Informatics",
     ],
 )

snakemake/__init__.py

@@ -2182,6 +2182,11 @@ def get_argument_parser(profile=None):
 
 def main(argv=None):
     """Main entry point."""
+
+    if sys.version_info < (3, 6):
+        print("Snakemake requires at least Python 3.6.", file=sys.stderr)
+        exit(1)
+
     parser = get_argument_parser()
     args = parser.parse_args(argv)
 

snakemake/common.py

@@ -10,6 +10,8 @@
 import inspect
 import uuid
 import os
+import asyncio
+import sys
 
 from ._version import get_versions
 
@@ -25,6 +27,15 @@
 ON_WINDOWS = platform.system() == "Windows"
 
 
+if sys.version_info < (3, 7):
+    def async_run(coroutine):
+        loop = asyncio.get_event_loop()
+        return loop.run_until_complete(coroutine)
+else:
+    async_run = asyncio.run
+
+
+
 class TBDInt(int):
     """An integer that prints into <TBD>"""
 

snakemake/io.py

@@ -29,8 +29,7 @@
 )
 from snakemake.logging import logger
 from inspect import isfunction, ismethod
-
-from snakemake.common import DYNAMIC_FILL, ON_WINDOWS
+from snakemake.common import DYNAMIC_FILL, ON_WINDOWS, async_run
 
 
 class Mtime:
@@ -134,7 +133,7 @@ def __init__(self, max_wait_time):
         self.max_wait_time = max_wait_time
 
     def mtime_inventory(self, jobs):
-        asyncio.run(self._mtime_inventory(jobs))
+        async_run(self._mtime_inventory(jobs))
 
     async def _mtime_inventory(self, jobs, n_workers=8):
         queue = asyncio.Queue()
@@ -252,7 +251,7 @@ def wrapper(self, *args, **kwargs):
         return wrapper
 
     def inventory(self):
-        asyncio.run(self._inventory())
+        async_run(self._inventory())
 
     async def _inventory(self):
         """Starting from the given file, try to cache as much existence and