Add __init__.py file in tests directory

This commit introduces an __init__.py file in the tests directory. The file is added to resolve a confusion caused by MyPype when encountering multiple files with the same name (conftest.py) in the tests directory. The issue is meditated by adding an __init__.py.

tests/__init__.py

@@ -0,0 +1,6 @@
+"""Test suite for segy.
+
+This file solely exists because MyPy gets confused by multiple
+files with the same name (conftest.py) exist in the tests directory.
+It doesn't when you add a __init__.
+"""