Break shell out of extension

[rogerbinns]

Currently the shell (python source) is included directly in the C extension via PyRun_StringFlags. This was desirable because it kept the binary as one file making deployment easier. However we are now a package and deployed via pypi so that doesn't matter. But there are also problems:

• 2k lines of python code are evaluated on loading the module (even if the shell is never used)
• any tracebacks say the file is <string>
• harder to do coverage testing
• type annotations aren't extracted automatically by mypy (it never loads the binary module)

[rogerbinns]

This is now complete