Bug 1680802: Install pylint requirements with legacy resolver r=firefox-build-system-reviewers,sheehan,glandium

pylint_requirements.txt fail to install with the new pip resolver due
to a conflict between astroid and lazy-object-proxy.
Rather than bumping those packages and handling the potential fallout,
the package-upgrade has been deferred and we will use the legacy
resolver in the interrim.

Differential Revision: https://phabricator.services.mozilla.com/D99940

python/mach_commands.py

@@ -242,7 +242,11 @@ class MachCommands(MachCommandBase):
                 and test["requirements"] not in installed_requirements
             ):
                 self.virtualenv_manager.install_pip_requirements(
-                    test["requirements"], quiet=True
+                    test["requirements"],
+                    quiet=True,
+                    # pylint_requirements.txt must use the legacy resolver until bug 1682959
+                    # is resolved.
+                    legacy_resolver=True,
                 )
                 installed_requirements.add(test["requirements"])