The [NLTK](https://pypi.org/project/nltk/) docs say it supports Python 3.7. However, when I try to install it with:

```
virtualenv -p python3.7 .env
. .env/bin/activate
pip install nltk

```

I get the error:

```
Collecting nltk
  Using cached https://files.pythonhosted.org/packages/f6/1d/d925cfb4f324ede997f6d47bea4d9babba51b49e87a767c170b77005889d/nltk-3.4.5.zip
    ERROR: Command errored out with exit status 1:
     command: /path/to/my/project/.env/bin/python3.7 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-12ut35ws/nltk/setup.py'"'"'; __file__='"'"'/tmp/pip-install-12ut35ws/nltk/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base pip-egg-info
         cwd: /tmp/pip-install-12ut35ws/nltk/
    Complete output (1 lines):
    error in nltk setup command: 'install_requires' must be a string or list of strings containing valid project/version requirement specifiers; Expected version spec in singledispatch; python_version < "3.4" at ; python_version < "3.4"
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

```

The error message sounds like NLTK only supports Python < 3.4, but that doesn't make any sense. What am I doing wrong?