Add compatibility with pip 10

poetry/repositories/legacy_repository.py

 from pip._vendor.pkg_resources import RequirementParseError
+
+try:
+    from pip._internal.exceptions import InstallationError
+    from pip._internal.req import InstallRequirement
+except ImportError:
+    from pip.exceptions import InstallationError
+    from pip.req import InstallRequirement
+
 from piptools.cache import DependencyCache
 from piptools.repositories import PyPIRepository
 from piptools.resolver import Resolver
@@ -167,9 +175,6 @@ class LegacyRepository(PyPiRepository):
         )
 
     def _get_release_info(self, name, version):  # type: (str, str) -> dict
-        from pip.req import InstallRequirement
-        from pip.exceptions import InstallationError
-
         ireq = InstallRequirement.from_line('{}=={}'.format(name, version))
         resolver = Resolver(
             [ireq], self._repository,

poetry/utils/pip_compat.py

-from pip import __version__
-
-from poetry.version import parse
-
-if parse(__version__) >= parse('9.0.2'):
-    from pip._internal.req import InstallRequirement
-    from pip._internal.utils.appdirs import user_cache_dir, user_config_dir
-else:
-    from pip.req import InstallRequirement
-    from pip.utils.appdirs import user_cache_dir, user_config_dir