CI: MyPy fixup
https://github.com/pandas-dev/pandas/runs/1064831108

```
mypy --version
mypy 0.782
Performing static analysis using mypy
pandas/io/common.py:168: error: unused 'type: ignore' comment
pandas/io/common.py:170: error: Incompatible default for argument "encoding" (default has type "None", argument has type "str")  [assignment]
pandas/io/common.py:172: error: Incompatible default for argument "mode" (default has type "None", argument has type "str")  [assignment]
Found 3 errors in 1 file (checked 1037 source files)
```