Add BUCK filename to Python in languages.yml

BUCK is the filename for the build files of the Facebook Buck build
system. BUCK files are valid Python files.

Eg.:
  https://github.com/GerritCodeReview/gerrit/blob/master/BUCK

Also add a missing sample for Pants / Bazel BUILD files. They are also
valid Python files.

samples/Python/filenames/BUILD

@@ -0,0 +1,45 @@
+package(default_visibility = ["//scripts/release:__pkg__"])
+
+filegroup(
+    name = "git",
+    srcs = glob([".git/**"]),
+)
+
+filegroup(
+    name = "dummy",
+    visibility = ["//visibility:public"],
+)
+
+filegroup(
+    name = "srcs",
+    srcs = glob(
+        ["**"],
+        exclude = [
+            "bazel-*/**",
+            "output/**",
+            ".*/**",
+        ],
+    ) + [
+        "//examples:srcs",
+        "//scripts:srcs",
+        "//site:srcs",
+        "//src:srcs",
+        "//tools:srcs",
+        "//third_party:srcs",
+    ],
+    visibility = ["//visibility:private"],
+)
+
+load("//tools/build_defs/pkg:pkg.bzl", "pkg_tar")
+
+pkg_tar(
+    name = "bazel-srcs",
+    files = [":srcs"],
+    strip_prefix = ".",
+    # Public but bazel-only visibility.
+    visibility = ["//:__subpackages__"],
+)
+
+load("//tools/build_rules/go:def.bzl", "go_prefix")
+
+go_prefix("github.com/bazelbuild/bazel")