Lingering "unclosed file" warnings

Created by: onyxfish

Was originally tracked as wireservice/agate#647, but really this was a csvkit issue. I've fixed most of them. Here are the remainder:

test_fixed_no_inference (tests.test_convert.test_fixed.TestFixed) ... /Users/cgroskopf/src/csvkit/csvkit/cli.py:124: ResourceWarning: unclosed file <_io.TextIOWrapper name='examples/testfixed_schema_no_inference.csv' mode='rt' encoding='utf-8'>
ok

test_invalid_options (tests.test_utilities.test_csvcut.TestCSVCut) ... /Users/cgroskopf/src/csvkit/csvkit/cli.py:294: ResourceWarning: unclosed file <_io.TextIOWrapper name='examples/dummy.csv' mode='rt' encoding='utf-8'>
ok

test_invalid_options (tests.test_utilities.test_csvgrep.TestCSVGrep) ... /Users/cgroskopf/src/csvkit/csvkit/cli.py:294: ResourceWarning: unclosed file <_io.TextIOWrapper name='examples/dummy.csv' mode='rt' encoding='utf-8'>
ok

test_duplicate_keys (tests.test_utilities.test_csvjson.TestCSVJSON) ... /Users/cgroskopf/src/csvkit/tests/test_utilities/test_csvjson.py:49: DeprecationWarning: Please use assertRaisesRegex instead.
ok

test_invalid_options (tests.test_utilities.test_csvstat.TestCSVStat) ... /Users/cgroskopf/src/csvkit/csvkit/cli.py:294: ResourceWarning: unclosed file <_io.TextIOWrapper name='examples/dummy.csv' mode='rt' encoding='utf-8'>
ok