Add explicit __init__.py to srcs to work around Python3 issues.

https://bugs.python.org/issue28424
https://bugs.python.org/issue28425
diff --git a/tests/BUILD b/tests/BUILD
index 0fe4656..bb2d53c 100644
--- a/tests/BUILD
+++ b/tests/BUILD
@@ -6,6 +6,10 @@
 
 package(default_visibility = ["//tests:__subpackages__"])
 
+exports_files([
+    "__init__.py",
+])
+
 load("//:subpar.bzl", "par_binary")
 
 # Utility targets
diff --git a/tests/package_a/BUILD b/tests/package_a/BUILD
index 589defd..da526ba 100644
--- a/tests/package_a/BUILD
+++ b/tests/package_a/BUILD
@@ -13,6 +13,9 @@
     name = "a_%s" % version,
     srcs = [
         "a.py",
+        "//:__init__.py",
+        "//tests:__init__.py",
+        "//tests/package_a:__init__.py",
     ],
     data = ["a_dat.txt"],
     default_python_version = version,
diff --git a/tests/package_b/BUILD b/tests/package_b/BUILD
index e222c8d..ffe5433 100644
--- a/tests/package_b/BUILD
+++ b/tests/package_b/BUILD
@@ -11,7 +11,12 @@
 
 [par_binary(
     name = "b_%s" % version,
-    srcs = ["b.py"],
+    srcs = [
+        "b.py",
+        "//:__init__.py",
+        "//tests:__init__.py",
+        "//tests/package_b:__init__.py",
+    ],
     data = ["b_dat.txt"],
     default_python_version = version,
     main = "b.py",