Periodic mode does not work

I recently got some SHT30 sensors and was trying to run them in periodic mode, but it crashes most of the time with this error:
```
File "/lib/adafruit_sht31d.py", line 380, in relative_humidity
File "/lib/adafruit_sht31d.py", line 262, in _read
File "/lib/adafruit_sht31d.py", line 245, in _data
File "adafruit_tca9548a.py", line 74, in readfrom_into
OSError: [Errno 19] No such device
```
Sometimes it works for sometimes before throwing this error. Single shot mode works perfectly fine.

Start by reproducing periodic mode with an SHT30 and compare it with single-shot mode. Trace the failing path through lib/adafruit_sht31d.py at lines 245, 262, and 380, then inspect adafruit_tca9548a.py line 74. Done means periodic reads continue without the reported OSError.

Written by the indexing model from the issue text.