Timeout for file flush

If connection to HSDS (or the internet) is lost during a file flush, the process hangs indefinitely. There should be some timeout logic, preferably set by a parameter to the flush function, that allows the HTTP request to stop after a certain amount of time.

On a device, like an IoT device, pulling the plug on the network connection mid-flush results in this sample script below hanging indefinitely.

```
import h5pyd

path = '/test/timeout.h5'

with h5pyd.File(path, 'w') as f:
dset = f.create_dataset('data', (1000000000,), dtype='i')
print('Start flush PUT')
f.flush()
print('Successful flush PUT')
```
Here is a full traceback after KeyboardInterrupt:
```
Start flush PUT
^CTraceback (most recent call last):
File "/home/user/repo/test_timeout.py", line 8, in
f.flush()
File "/home/user/.local/lib/python3.10/site-packages/h5pyd/_hl/files.py", line 775, in flush
rsp = self.PUT(req, body=body)
File "/home/user/.local/lib/python3.10/site-packages/h5pyd/_hl/base.py", line 1017, in PUT
rsp = self._id._http_conn.PUT(req, body=body, params=params, format=format)
File "/home/user/.local/lib/python3.10/site-packages/h5pyd/_hl/httpconn.py", line 580, in PUT
rsp = s.put(
File "/home/user/.local/lib/python3.10/site-packages/requests/sessions.py", line 649, in put
return self.request("PUT", url, data=data, **kwargs)
File "/home/user/.local/lib/python3.10/site-packages/requests/sessions.py", line 589, in request
resp = self.send(prep, **send_kwargs)
File "/home/user/.local/lib/python3.10/site-packages/requests/sessions.py", line 703, in send
r = adapter.send(request, **kwargs)
File "/home/user/.local/lib/python3.10/site-packages/requests/adapters.py", line 644, in send
resp = conn.urlopen(
File "/home/user/.local/lib/python3.10/site-packages/urllib3/connectionpool.py", line 787, in urlopen
response = self._make_request(
File "/home/user/.local/lib/python3.10/site-packages/urllib3/connectionpool.py", line 534, in _make_request
response = conn.getresponse()
File "/home/user/.local/lib/python3.10/site-packages/urllib3/connection.py", line 565, in getresponse
httplib_response = super().getresponse()
File "/usr/lib/python3.10/http/client.py", line 1375, in getresponse
response.begin()
File "/usr/lib/python3.10/http/client.py", line 318, in begin
version, status, reason = self._read_status()
File "/usr/lib/python3.10/http/client.py", line 279, in _read_status
line = str(self.fp.readline(_MAXLINE + 1), "iso-8859-1")
File "/usr/lib/python3.10/socket.py", line 705, in readinto
return self._sock.recv_into(b)
File "/usr/lib/python3.10/ssl.py", line 1274, in recv_into
return self.read(nbytes, buffer)
File "/usr/lib/python3.10/ssl.py", line 1130, in read
return self._sslobj.read(len, buffer)
KeyboardInterrupt
```

Since you are using the requests library, which as far as I understand already have a timeout parameter, which you can add in h5pyd/_hl/httpconn.py", line 580, in PUT, rsp = s.put(, you can hopefully add timeout as a parameter in file.flush() and pass it to the put request.