Use openfst_python to replace pywrapfst

Looks like we could use https://pypi.org/project/openfst-python to replace pywrapfst. The library is installable through pip without compilation on Python 3.7

1. add in requirements.txt
2. Replace pyrapfst calls by openfst_python
3. Replace VectorFst by the equivalent in this lib
4. compare performance with master
5. if performance are similar, buildn fst high-level library that can pick between openfst official library and this one

try:
  import pywrapfst as fst
  FST_LIB = "pywrapfst"
except ImportError
  try:
    import openfst_python as fst
    FST_LIB = "openfst_python"
  except ImportError
    FST_LIB = None  

def do_something():
  if FST_LIB == "..."
     do_A():
...