Fix type checking with latest Mypy (0.770)

Author: bbc2

src/dotenv/main.py

@@ -275,6 +275,7 @@ def _is_interactive():
             current_file = __file__
 
         while frame.f_code.co_filename == current_file:
+            assert frame.f_back is not None
             frame = frame.f_back
         frame_filename = frame.f_code.co_filename
         path = os.path.dirname(os.path.abspath(frame_filename))

tox.ini

@@ -22,7 +22,7 @@ commands =
 skip_install = true
 deps =
   flake8
-  mypy==0.761
+  mypy
 commands =
   flake8 src tests
   mypy --python-version=3.8 src tests