change http client library

[dogTK]

issue

When launching generate_tiny_10x_pbmc.py, which is located under CellBender/examples/remove_background/, an error occurs due to the dataset not being downloaded.

(base) hoge@dogTK remove_background % pwd
/Users/hoge/Desktop/CellBender/examples/remove_background
(base) hoge@dogTK remove_background % python generate_tiny_10x_pbmc.py
Downloading pbmc4k (CellRanger 2.1.0, v2 Chemistry)...
Could not retrieve pbmc4k (CellRanger 2.1.0, v2 Chemistry) -- cannot continue!
(base) hoge@dogTK remove_background % 

change point

change the http-clinet library from urllib.request to requests.

Things Checked

I checked to download the sample file.

[sjfleming]

Thank you so much for contributing this @dogTK , but for now, I am going to close this PR in favor of the way it has been addressed in v0.3.0 and mentioned in #95

But if the fix in v0.3.0 ends up not working well, then I might reopen this and use your fix instead.

[sjfleming]

Addressed in #238