SystemError in find_common_type()
I have a generator that return slices of a pandas dataframe for use in a TensorFlow dataset. When running my script in debug mode, I get the following trace:

```
Exception ignored in: <generator object find_common_type.<locals>.<genexpr> at 0x11a367a40>
Traceback (most recent call last):
  File "/Users/harald/anaconda3/envs/thesis/lib/python3.6/site-packages/pandas/core/dtypes/cast.py", line 1074, in <genexpr>
    if all(is_dtype_equal(first, t) for t in types[1:]):
SystemError: error return without exception set
Exception ignored in: <generator object find_common_type.<locals>.<genexpr> at 0x11a367a40>
Traceback (most recent call last):
  File "/Users/harald/anaconda3/envs/thesis/lib/python3.6/site-packages/pandas/core/dtypes/cast.py", line 1081, in <genexpr>
    if all(is_datetime64_dtype(t) for t in types):
SystemError: error return without exception set
Exception ignored in: <generator object find_common_type.<locals>.<genexpr> at 0x11a367a40>
Traceback (most recent call last):
  File "/Users/harald/anaconda3/envs/thesis/lib/python3.6/site-packages/pandas/core/dtypes/cast.py", line 1083, in <genexpr>
    if all(is_timedelta64_dtype(t) for t in types):
SystemError: error return without exception set
Exception ignored in: <generator object _yield_value at 0x11ba350f8>
Traceback (most recent call last):
  File "/Users/harald/anaconda3/envs/thesis/lib/python3.6/site-packages/tensorflow/python/data/util/nest.py", line 95, in _yield_value
    yield iterable[key]
SystemError: error return without exception set
Exception ignored in: <generator object _yield_value at 0x11ba35f68>
Traceback (most recent call last):
  File "/Users/harald/anaconda3/envs/thesis/lib/python3.6/site-packages/tensorflow/python/data/util/nest.py", line 100, in _yield_value
    yield value
SystemError: error return without exception set
```
This does not happen when the generator is called from the main python script rather than from TensorFlow, so it is hard to know exactly what goes wrong here, or if it is an issue with TensorFlow or Pandas.