Fix up versioning

[neworderofjamie]

• Single text file in repository root with version string
• Read into version in setup.py for both mlGeNN and mlGeNN TF

# Read version from txt file
with open(os.path.join(genn_path, "version.txt")) as version_file:
    version = version_file.read().strip()

• Export version through __version__ in init.py in mlGeNN and mlGeNN TF

if sys.version_info >= (3, 8):
    from importlib import metadata
else:
    import importlib_metadata as metadata

__version__ = metadata.version("pygenn")