[urllib imports] Multiple urllib imports are missing

[frosencrantz]

Small description
Missing imports for urllib.error and urllib.parse

Expected result
No stack traces from missing imports.

Actual result with screenshot

First Trace

Traceback (most recent call last):
  File "/usr/lib/python3.9/site-packages/visidata/threads.py", line 200, in _toplevelTryFunc
    t.status = func(*args, **kwargs)
  File "/usr/lib/python3.9/site-packages/visidata/threads.py", line 219, in _func
    func(*args, **kwargs)
  File "/usr/lib/python3.9/site-packages/visidata/plugins.py", line 97, in reload
    except urllib.error.URLError as e:
AttributeError: module 'urllib' has no attribute 'error'

Second trace

Traceback (most recent call last):
...
  File "/usr/lib/python3.9/site-packages/visidata/loaders/http.py", line 50, in _iter_lines
    links = urllib.parse.parse_header(linkhdr)
AttributeError: module 'urllib.parse' has no attribute 'parse_header'

Reproduce:

vd https://blog.nature.org/2021/04/12/beaver-otter-muskrat-a-field-guide-to-freshwater-mammals/

Please attach the commandlog (saved with Ctrl-D) to show the steps that led to the issue.
See here for more details.

Additional context
Please include the version of VisiData. Latest develop version of VisData. Python 3.9.2