MAINT Fixes for Python scripts

[rth]

A few fixes to python scripts,

• add Python 3 support (Python 2 is deprecated at the end of the year), Python2 will still work. The main difference comes from the fact that,

  $ python2 -c "print(len('×'))"
  2
  $ python3 -c "print(len('×'))"                               
  1

• fix invalid escape characters in strings e.g. "\w" -> r"\w"
• fix the download URL to get Unicode 9.0 used here, otherwise data for Unicode 12 is downloaded as mentioned in Unicode sentence boundaries #24 (review)

I double checked that these scripts generate the included tables.rs and testdata.rs both with Python 2 and 3

[Manishearth]

Looks good, needs conflicts to be resolved

[Manishearth]

Thanks!