Fix bug with doctest references

[Erotemic]

Currently hitting a pypy issue with the Jupyter notebook tests. Not sure what the cause is.

Getting:

(venv) root@454e785b355d:~/test-minimal-pypy3#     xdoctest testing/test_notebook.py test_xdoctest_inside_notebook

=====================================
_  _ ___  ____ ____ ___ ____ ____ ___
 \/  |  \ |  | |     |  |___ [__   |
_/\_ |__/ |__| |___  |  |___ ___]  |

=====================================

Start doctest_module('testing/test_notebook.py')
Listing tests
gathering tests
running 1 test(s)
====== <exec> ======
* ZERO-ARG FUNC : testing/test_notebook.py::test_xdoctest_inside_notebook:0
ZERO-ARG SOURCE
1 >>> test_xdoctest_inside_notebook()
ZERO-ARG STDOUT/STDERR
executing notebook in dpath = '/root/test-minimal-pypy3/testing'
[IPKernelApp] ERROR | Invalid Message
Traceback (most recent call last):
  File "/root/test-minimal-pypy3/venv/site-packages/ipykernel/kernelbase.py", line 227, in dispatch_shell
    msg = self.session.deserialize(msg, content=True, copy=False)
  File "/root/test-minimal-pypy3/venv/site-packages/jupyter_client/session.py", line 924, in deserialize
    self._add_digest(signature)
  File "/root/test-minimal-pypy3/venv/site-packages/jupyter_client/session.py", line 867, in _add_digest
    self.digest_history.add(signature)
TypeError: unhashable type: '_cffi_backend.buffer'
[IPKernelApp] ERROR | Invalid Message
Traceback (most recent call last):
  File "/root/test-minimal-pypy3/venv/site-packages/ipykernel/kernelbase.py", line 227, in dispatch_shell
    msg = self.session.deserialize(msg, content=True, copy=False)
  File "/root/test-minimal-pypy3/venv/site-packages/jupyter_client/session.py", line 920, in deserialize
    if signature in self.digest_history:
TypeError: unhashable type: '_cffi_backend.buffer'
[IPKernelApp] ERROR | Invalid Message
Traceback (most recent call last):
  File "/root/test-minimal-pypy3/venv/site-packages/ipykernel/kernelbase.py", line 227, in dispatch_shell
    msg = self.session.deserialize(msg, content=True, copy=False)
  File "/root/test-minimal-pypy3/venv/site-packages/jupyter_client/session.py", line 920, in deserialize
    if signature in self.digest_history:
TypeError: unhashable type: '_cffi_backend.buffer'

[codecov]

Codecov Report

Merging #90 (4949f7e) into master (b807cdf) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   84.94%   84.84%   -0.10%     
==========================================
  Files          22       22              
  Lines        2723     2706      -17     
  Branches      579      575       -4     
==========================================
- Hits         2313     2296      -17     
+ Misses        320      319       -1     
- Partials       90       91       +1