`pubmed_qa` checksum mismatch
Hi @jon-tow, thanks for reporting.

This issue was already reported and its root cause is a change in the Google Drive service. See:
- #3786 

We have already fixed it. See:
- #3787 

We are planning to make a patch release today.

In the meantime, you can get this fix by installing our library from the GitHub master branch:
```shell
pip install git+https://github.com/huggingface/datasets#egg=datasets
```
Then, if you had previously tried to load the data and got the checksum error, you should force the redownload of the data (before the fix, you just downloaded and cached the virus scan warning page, instead of the data file):
```shell
load_dataset("...", download_mode="force_redownload")
```
## Describe the bug
Loading [`pubmed_qa`](https://huggingface.co/datasets/pubmed_qa) results in a mismatched checksum error.

## Steps to reproduce the bug
```python
# Sample code to reproduce the bug
import datasets
try:
    datasets.load_dataset("pubmed_qa", "pqa_labeled")
except Exception as e:
    print(e)

try:
    datasets.load_dataset("pubmed_qa", "pqa_unlabeled")
except Exception as e:
    print(e)

try:
    datasets.load_dataset("pubmed_qa", "pqa_artificial")
except Exception as e:
    print(e)
```

## Expected results
Successful download.

## Actual results
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.9/site-packages/datasets/load.py", line 1702, in load_dataset
    builder_instance.download_and_prepare(
  File "/usr/local/lib/python3.9/site-packages/datasets/builder.py", line 594, in download_and_prepare
    self._download_and_prepare(
  File "/usr/local/lib/python3.9/site-packages/datasets/builder.py", line 665, in _download_and_prepare
    verify_checksums(
  File "/usr/local/lib/python3.9/site-packages/datasets/utils/info_utils.py", line 40, in verify_checksums
    raise NonMatchingChecksumError(error_msg + str(bad_urls))
datasets.utils.info_utils.NonMatchingChecksumError: Checksums didn't match for dataset source files:
['https://drive.google.com/uc?export=download&id=1RsGLINVce-0GsDkCLDuLZmoLuzfmoCuQ', 'https://drive.google.com/uc?export=download&id=15v1x6aQDlZymaHGP7cZJZZYFfeJt2NdS']

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.18.3
- Platform: macOS
- Python version: 3.8.1
- PyArrow version: 3.0.0
