fix a runtime error on Raspberry pi zero

[hyansuper]

I have a Raspberry pi zero w(Buster), it imports pulseio without error, but it seems to be unable to create a PulseIn at this line and produce a runtime error, if I change _USE_PULSEIO to False manually, the problem's gone

[hyansuper]

Sorry, this is only a quick fix for me, not a good solution.
the source of the error is in blinka:

Python 3.7.3 (default, Dec 20 2019, 18:57:59)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import board, pulseio
>>> p = pulseio.PulseIn(board.D14)
/usr/local/lib/python3.7/dist-packages/adafruit_blinka/microcontroller/bcm283x/pulseio/libgpiod_pulsein: error while loading shared libraries: libgpiod.so.2: cannot open shared object file: No such file or directory
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/dist-packages/adafruit_blinka/microcontroller/bcm283x/pulseio/PulseIn.py", line 64, in __init__
    message = self._wait_receive_msg()
  File "/usr/local/lib/python3.7/dist-packages/adafruit_blinka/microcontroller/bcm283x/pulseio/PulseIn.py", line 80, in _wait_receive_msg
    raise RuntimeError("Timed out waiting for PulseIn message")
RuntimeError: Timed out waiting for PulseIn message