Fix warnings with Python 3.7

[dmerejkowsky]

Before:

$ pytest
src/poetry/poetry/utils/appdirs.py:92: DeprecationWarning: invalid escape sequence \D
  """
src/poetry/poetry/utils/appdirs.py:154: DeprecationWarning: invalid escape sequence \D
  """
...
322 passed, 42 warnings in 8.24 seconds

After:

$ pytest
...
322 passed, 2 warnings in 5.08 seconds

Note: the remaining 2 warnings are coming from jsonschema and html5lib, we can probably ignore them for now. (Maybe I'll open an issue upstream)

Cheers!

[dmerejkowsky]

By the way, would you be willing to accept a PR adding support for Python3.7 to the CI ?

Update: I've done it already :)

[sdispater]

Thanks!

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.