Rename VERSION to VERSION.txt

Installing python package fails with below error.
This change addresses this issue
```
  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [10 lines of output]
      Traceback (most recent call last):
        File "<string>", line 2, in <module>
        File "<pip-setuptools-caller>", line 34, in <module>
        File "/home/alferre/code/sentencepiece/python/setup.py", line 111, in <module>
          version=version(),
        File "/home/alferre/code/sentencepiece/python/setup.py", line 36, in version
          with codecs.open('VERSION.txt', 'r', 'utf-8') as f:
        File "/opt/conda/envs/ptca/lib/python3.8/codecs.py", line 905, in open
          file = builtins.open(filename, mode, buffering)
      FileNotFoundError: [Errno 2] No such file or directory: 'VERSION.txt'
      [end of output]
```

0 files changed, 0 insertions, 0 deletions