find_dotenv() fails when it is the target of a thread

Invoking load_dotenv as the target of a thread effectively does nothing because find_dotenv is unable to locate the local .env file.

import dotenv
threading.Thread(target=dotenv.load_dotenv).start()

This is because find_dotenv() walks the stack frames of its callers to figure out where the .env file might be located, but when load_dotenv() is the top frame in the call stack, find_dotenv cannot figure out where .env is. A simple workaround is to wrap load_dotenv in another function.

def load():
  dotenv.load_dotenv()
threading.Thread(target=load).start()

Start with the find_dotenv and load_dotenv entry points described in the issue, and reproduce the threaded call with a local .env file. Trace how find_dotenv identifies the caller when load_dotenv is the thread target. Done means the threaded invocation locates the local file and has regression coverage.