move isoformat to compat, allow date, misc fixes

my/core/compat.py

@@ -2,6 +2,8 @@
 Some backwards compatibility stuff/deprecation helpers
 '''
 from types import ModuleType
+from typing import Callable
+from datetime import datetime
 
 from . import warnings
 from .common import LazyLogger
@@ -10,6 +12,16 @@ from .common import LazyLogger
 logger = LazyLogger('my.core.compat')
 
 
+fromisoformat: Callable[[str], datetime]
+import sys
+if sys.version_info[:2] >= (3, 7):
+    # prevent mypy on py3.6 from complaining...
+    fromisoformat_real = datetime.fromisoformat
+    fromisoformat = fromisoformat_real
+else:
+    from .py37 import fromisoformat
+
+
 def pre_pip_dal_handler(
         name: str,
         e: ModuleNotFoundError,