Added path separator dependent on system (#53)

defs.bzl

diff --git a/defs.bzl b/defs.bzl
index cb1375f..bbac671 100644
--- a/defs.bzl
+++ b/defs.bzl
@@ -22,7 +22,8 @@
         rctx.path(Label("@" + repo + "//:BUILD.bazel")).dirname
         for repo in all_requirements
     ]
-    pypath = ":".join([str(p) for p in [rules_root] + thirdparty_roots])
+    separator = ":" if not "windows" in rctx.os.name.lower() else ";"
+    pypath = separator.join([str(p) for p in [rules_root] + thirdparty_roots])
 
     args = [
         python_interpreter,