Properties missing from FileHandler

Everything here ought to be legal...

```
from logging import FileHandler
fh = FileHandler('foo')
print(fh.baseFilename)
print(fh.mode)
```

... but mypy disagrees:

```
$ mypy ~/test.py
test.py:3: error: "FileHandler" has no attribute "baseFilename"
test.py:4: error: "FileHandler" has no attribute "mode"
$ 
```