lib fails on async requests

[volkerrichert]

If your are using this lib with in async and parallel calls some requests fails because they cannot connect correctly ans fails with "Max retries reached"-SendError exception.

I implement a ThreadSafeE3DC(E3DC) class with SendRequest is syncronized by a lock. I'll like to create a PR, but which solution is prefered?

• an extra class TreadSafeE3DC
• a patched version of the E3DC class?

Both makes sense in some cases.

Volker