Thanks to dglaude for this update!
You must update your import statements with the new version of the library.
from adafruit_pm25.i2c import PM25_I2C
or
from adafruit_pm25.uart import PM25_UART
See the example script.
To use in CircuitPython, simply install the [Adafruit CircuitPython bundle](https://circuitpython.org/libraries).
To use in CPython, `pip3 install adafruit-circuitpython-pm25`.
Read the [docs](http://circuitpython.readthedocs.io/projects/pm25/en/latest/) for info on how to use it.