As you said, get_chart is defined in blockchain.statistics, but importing the statistics module does bring its members into the global namespace. You have to dot off of it to access its members, such as get_chart:
from blockchain import statistics
Alternatively you can import the function directly:
from blockchain.statistics import get_chart
Unfortunately, that won't solve the larger issue at hand which is that the repository for the package appears to be abandoned. For your request, it tries to access data from the URL https://blockchain.info/charts/mempool-size?format=json×pan=1year, which results from it downloading an HTML page instead of JSON.
Still, you can access the charts API using the docs provided here: https://www.blockchain.com/api/charts_api
For your request, the correct URL to use is: https://api.blockchain.info/charts/mempool-size?format=json×pan=1year
You can download it and parse the JSON into a dictionary like so:
urllib.request import urlopen url = 'https://api.blockchain.info/charts/mempool-size?format=json×pan=1year'
data = json.loads(urlopen(url).read())