Fix crash in --version

$ python3 tinyprog --version
Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "tinyprog/__main__.py", line 474, in <module>
    main()
  File "tinyprog/__main__.py", line 283, in main
    tinyprog.__version__, tinyprog.__full_version__))
AttributeError: module 'tinyprog' has no attribute '__version__'

Author: mbuesch

programmer/tinyprog/__init__.py

@@ -18,7 +18,7 @@
     __version__ = get_distribution(__name__).version
 except DistributionNotFound:
     # package is not installed
-    pass
+    __version__ = "unknown"
 
 try:
     from .full_version import __full_version__