Downgrade pyext dependency

[shehrozek-cerebras]

Currently, the pyext package dependency is pinned to version 0.7. However, this yields the following error with Python 3.11 when trying to install bigcode-eval package via pip:

oargspec = inspect.getargspec
           ^^^^^^^^^^^^^^^^^^
      AttributeError: module 'inspect' has no attribute 'getargspec'. Did you mean: 'getargs'?

pyext==0.7 doesn't look like an official release version (https://pypi.org/project/pyext/#history) and so it doesn't seem like this issue will be fixed any time soon to work with the newer version of Python

So in order to make bigcode-eval work with Python 3.11, this change downgrades the pyext package pin to 0.5. Note that there was a PR (#181) raised to address this issue previously, but not sure why it got closed.

[shehrozek-cerebras]

CC: @loubnabnl for reviews. Thanks!

[loubnabnl]

LGTM, thanks for the PR!