Update ipdb to 0.9.4

[pyup-bot]

There's a new version of ipdb available.
You are currently using 0.8.1. I have updated it to 0.9.4

Changelog

0.9.4

---

• Fix Restart error when using python -m ipdb
  [gotcha]

0.9.3

---

• Don't require users to pass a traceback to post_mortem.
  [Wilfred]

0.9.2

---

• Closes Using set_trace in 0.9.1 raises an error gotcha/ipdb#93.
  [gotcha]

0.9.1

---

• Reset sys.modules['__main__'] to original value.
  Closes ipdb.set_trace() breaks inspect gotcha/ipdb#85
  [gotcha]
• Fix support of IPython versions 0.x
  [asivokon]

0.9.0

---

• Switch to revised BSD license (with approval of all contributors).
  Closes Relicense as PSF-compatible? gotcha/ipdb#68
  [lebedov, gotcha]

0.8.3

---

• Don't pass sys.argv to IPython for configuration.
  [emulbreh]

0.8.2

---

• Fix lexical comparison for version numbers.
  [sas23]
• Allow configuring how many lines of code context are displayed
  by set_trace
  [JamshedVesuna]
• If an instance of IPython is already running its configuration will be
  loaded.
  [IxDay]

Happy merging! 🤖

[codecov-io]

Current coverage is 100%

Merging #79 into master will not change coverage

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[pyup-bot]

Closing this in favor of #80