Add .bzl extension to Python in languages.yml

.bzl is the extension for Skylark, a subset of Python that is used
to define build extensions for the Bazel build system.

Eg:
  https://github.com/bazelbuild/bazel/blob/master/tools/build_rules/closure/closure_js_binary.bzl

lib/linguist/languages.yml

@@ -2822,6 +2822,7 @@ Python:
   color: "#3572A5"
   extensions:
   - .py
+  - .bzl
   - .cgi
   - .fcgi
   - .gyp