[Python] NativeFile.write shouldn't accept unicode strings

[asfimport]

Arrow files are binary, but for some reason NativeFile.write silently converts unicode strings to bytes.

>>> b = io.BytesIO()
>>> b.write("foo")
Traceback (most recent call last):
  File "<ipython-input-6-a7195dbc0372>", line 1, in <module>
    b.write("foo")

TypeError: a bytes-like object is required, not 'str'
>>> f = pa.PythonFile(b)
>>> f.write("foo")
>>> b.getvalue()
b'foo'
>>> f.write("😀")
>>> b.getvalue()
b'foo\xf0\x9f\x98\x80'

Reporter: Antoine Pitrou / @pitrou
Assignee: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #2570

_{Note: This issue was originally created as ARROW-3227. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Agreed. I don't think this is so on purpose

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 2570
#2570