Type-annotated "encoding" field crashes isort

[srittau]

This came up while trying to enforce isort imports in typeshed. Please consider the following file:

class X:
    encoding: str

(The actual file it stumbles over is stdlib/3/io.pyi.)

Running isort 4.3.21 with Python 3.7 against this fails with the following traceback:

Traceback (most recent call last):
  File "./.venv/bin/isort", line 10, in <module>
    sys.exit(main())
  File "/home/srittau/.venv/lib/python3.7/site-packages/isort/main.py", line 379, in main
    for sort_attempt in attempt_iterator:
  File "/home/srittau/.venv/lib/python3.7/site-packages/isort/main.py", line 377, in <genexpr>
    attempt_iterator = (sort_imports(file_name, **arguments) for file_name in file_names)
  File "/home/srittau/.venv/lib/python3.7/site-packages/isort/main.py", line 88, in sort_imports
    result = SortImports(file_name, **arguments)
  File "/home/srittau/.venv/lib/python3.7/site-packages/isort/isort.py", line 114, in __init__
    with io.open(file_path, encoding=file_encoding, newline='') as file_to_import_sort:
LookupError: unknown encoding: str

[srittau]

Correction: The file that triggered the original problem was stdlib/3/encodings/__init__.pyi.

[hauntsaninja]

Hello! Would it be possible to cut a new release that includes this fix?