Intermittent unit test crashes

[pieths]

The Windows unit tests are randomly crashing during the CI runs. It does not appear to happen locally (Windows + Python3.7). See this pull request for more information.

The following tests seem to crash most frequently:

• TestFromKey::test_example_key_to_text_typeerror_u4
• TestFromKey::test_fromkey_multiple_columns
• TestSyntaxExpectedFailures::test_syntax_ambiguities_y
• test_data_with_missing.py::TestDataWithMissing::test_null
• TestSyntaxOneHotVectorizer::test_syntax5_failing
• TestDataWithMissing::test_input_conversion_to_float

Here is output from a recent run that contains two crashes. log_63_1887.zip