Fix gf shell not being able to access globals inside user defined functions.

[antoinevg]

This PR fixes an issue introduced in #414 where executing the following code in the GreatFET shell will fail with a NameError: name 'time' is not defined error:

import time
def read(x):
    time.sleep(x)
    print("Done")
read(2)

The fix has been tested on IPython versions 8.12.3 and 8.26.0.

This PR also rolls back the version of IPython specified in the package dependencies to "ipython~=8.12" as this was the final version supported by Python 3.8 which is our minimum Python specification for the package.

The current latest version IPython is 8.26.0 which requires Python >=3.10.

If GreatFET is installed on systems with Python >=3.10 then the dependency spec ~=8.12 will resolve as: >=8.12, ==8.* .

This ensures that the latest IPython will be installed on those systems.

---

Closes #435

[mossmann]

I love it!