Fix acceptance tests on newer versions of setuptools

Since setuptools v49, acceptance tests were failing to import pynixify.
After a long session of debugging, the problem turned out to be that I
needed to add a __init__.py file.

Root cause of the regression: pypa/setuptools#2165

acceptance_tests/__init__.py

@@ -0,0 +1,16 @@
+# pynixify - Nix expression generator for Python packages
+# Copyright (C) 2020 Matías Lang
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+