make sure the --before command line argument is respected when generating weekly updates

[grlee77]

Current behavior seems to be not filtering at all for the before date.

The issue was encountered when trying to generate a weekly update for Feb 1st through the 7th.
python skimage_weekly_update.py --before 2021-02-08 --after 2021-02-01

but this was also including all issues/PRs up until the current date. After the modifications here, the report looks as expected.

[pep8speaks]

Hello @grlee77! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file skimage_weekly_update.py:

Line 72:80: E501 line too long (80 > 79 characters)
Line 78:13: E128 continuation line under-indented for visual indent
Line 82:13: E128 continuation line under-indented for visual indent
Line 87:13: E128 continuation line under-indented for visual indent
Line 87:80: E501 line too long (86 > 79 characters)
Line 99:13: E128 continuation line under-indented for visual indent
Line 106:13: E128 continuation line under-indented for visual indent

[jni]

Yeah that was my bad. 😬 Thanks for the fix!