Suppress deprecation warnings in html5lib (#688)

html5lib currently issues a DeprecationWarning in python ^3.7. This
warning is fixed here:

https://github.com/html5lib/html5lib-python/commit/4f9235752cea29c5a31721440578b430823a1e69

I have spoken with the maintainers (#whatwg on freenode) and they said
that a new release of html5lib is not scheduled as the package is not
actively maintained, but it sounded like they were aware of the issue
and could release a new version. (I did not press further.)

This is a very minor change with no major intended side effects, other
than that it cleans up output while running tests (and presumably from
any clients using the legacy_repository type).

poetry/repositories/legacy_repository.py

@@ -21,7 +21,6 @@ from typing import Generator
 from typing import Optional
 from typing import Union
 
-import html5lib
 import requests
 
 from cachecontrol import CacheControl
@@ -47,6 +46,12 @@ from .auth import Auth
 from .exceptions import PackageNotFound
 from .pypi_repository import PyPiRepository
 
+import warnings
+
+with warnings.catch_warnings():
+    warnings.simplefilter("ignore")
+    import html5lib
+
 
 class Page: