build: add version.py for generating version.h

version.h is essential for building, and its generation was done by a
shell script. Strictly speaking, python should in general be more
portable (windows), and would be better for the upcoming meson build to
simply just execute a python script. version.py has some small
differences with version.sh which shouldn't matter but they are noted
below.

- version.sh accepted several arguments that seemed useless (like
  --cwd). These were removed from version.py.
- version.py takes either no arguments (prints the version) or it takes
  exactly one argument specifying the complete path of where the header
  should be generated.
- version.sh attempted to read a file named "snapshot_version". The
  comments noted that this was for "daily tarball snapshots". Such a
  file does not exist in the source tree, and it's not really clear that
  anyone actually uses this. This logic was removed from version.py.
- version.py reads the SOURCE_DATE_EPOCH environment variable. Some
  distros use this for reproducible builds. Technically you could also
  just disable the build date but this is only a couple of extra lines
  and maybe it's prettier than UNKNOWN.
- version.py also doesn't attempt to display timezone information in the
  build date. It only shows UTC time.

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