[Python] Fix suppressed exception in ParquetWriter.__del__

[asfimport]

pyarrow-test-2.7/lib/python2.7/site-packages/pyarrow/tests/test_parquet.py::test_coerce_timestamps Exception AttributeError: "'ParquetWriter' object has no attribute 'is_open'" in <bound method ParquetWriter.__del__ of <pyarrow.parquet.ParquetWriter object at 0x11bc1f4d0>> ignored

Reporter: Wes McKinney / @wesm

PRs and other links:

• GitHub Pull Request #1286

_{Note: This issue was originally created as ARROW-1768. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 1286
#1286