fix type error, pin mypy

Author: willmcgugan

fs/tools.py

@@ -15,7 +15,7 @@
 from .path import recursepath
 
 if typing.TYPE_CHECKING:
-    from typing import IO, List, Optional, Text
+    from typing import IO, List, Optional, Text, Union
     from .base import FS
 
 
@@ -52,7 +52,9 @@ def copy_file_data(src_file, dst_file, chunk_size=None):
     read = src_file.read
     write = dst_file.write
     # The 'or None' is so that it works with binary and text files
-    for chunk in iter(lambda: read(_chunk_size) or None, None):
+    for chunk in iter(
+        lambda: read(_chunk_size) or None, None
+    ):  # type: Optional[Union[bytes, str]]
         write(chunk)
 
 

tox.ini

@@ -10,7 +10,7 @@ commands = coverage run -m pytest --cov-append {posargs} {toxinidir}/tests
 [testenv:typecheck]
 python = python37
 deps =
-  mypy
+  mypy==0.740
   -r {toxinidir}/testrequirements.txt
 commands = make typecheck
 whitelist_externals = make