Cannot define csp nodes/graphs in raw python interpreter

[timkpaine]

reported in #88 (comment)

Python 3.8.10 | packaged by conda-forge | (default, Sep 13 2021, 21:46:58)
[GCC 9.4.0] on linux
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>>> import csp
>>> import pandas
>>> from datetime import datetime
>>> @csp.graph
... def foo(x: pandas.DataFrame):
...   pass
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
  File "/csp/csp/impl/wiring/graph.py", line 393, in graph
    return _impl(func)
  File "/csp/csp/impl/error_handling.py", line 37, in __exit__
    raise exc_val.with_traceback(new_tb)
OSError: could not get source code
>>>