Fix considering py_proto_library (#59)

With my change removing the specific rule types, some common rules broke
because of missing the attrs later. This seems like a good one to key on
to ignore

mypy/private/mypy.bzl

diff --git a/mypy/private/mypy.bzl b/mypy/private/mypy.bzl
index 447ba15..fc40283 100644
--- a/mypy/private/mypy.bzl
+++ b/mypy/private/mypy.bzl
@@ -72,6 +72,10 @@
     if not _opt_in(ctx.attr._opt_in_tags, ctx.rule.attr.tags):
         return []
 
+    # ignore rules that don't carry source files like py_proto_library
+    if not hasattr(ctx.rule.files, "srcs"):
+        return []
+
     # we need to help mypy map the location of external deps by setting
     # MYPYPATH to include the site-packages directories.
     external_deps = []