Fix allow_origin_pat property to properly parse regex

[havok2063]

This PR fixes a bug with the allow_origin_pat which prevented the Jupyter server from recognizing and parsing regex patterns. Follows from discussions with @mariobuikhuizen on mariobuikhuizen/voila-embed#24. Example traceback of the current broken implementation that this PR fixes is

Traceback (most recent call last):
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/tornado/http1connection.py", line 273, in _read_message
    delegate.finish()
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/tornado/routing.py", line 268, in finish
    self.delegate.finish()
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/tornado/web.py", line 2290, in finish
    self.execute()
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/tornado/web.py", line 2309, in execute
    self.handler = self.handler_class(
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/tornado/web.py", line 227, in __init__
    self.clear()
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/tornado/web.py", line 328, in clear
    self.set_default_headers()
  File "/Users/bcherinka/anaconda3/envs/havokve/lib/python3.8/site-packages/jupyter_server/base/handlers.py", line 300, in set_default_headers
    if origin and self.allow_origin_pat.match(origin):
AttributeError: 'str' object has no attribute 'match'

[Zsailer]

Thanks, @havok2063.

Because allow_origin_pat is a configurable unicode trait in the main ServerApp, I think we should preserve its type in this property as a unicode string. Instead, I suggest we replace all instances of allow_origin_pat.match(...) with re.match(allow_origin_pat, ...)

[codecov-commenter]

Codecov Report

Merging #603 (b82c6fd) into master (fda4cc5) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head b82c6fd differs from pull request most recent head 516f92d. Consider uploading reports for the commit 516f92d to get more accurate results

@@            Coverage Diff             @@
##           master     #603      +/-   ##
==========================================
- Coverage   77.40%   77.35%   -0.06%     
==========================================
  Files         110      110              
  Lines       10251    10252       +1     
  Branches     1258     1258              
==========================================
- Hits         7935     7930       -5     
- Misses       1918     1926       +8     
+ Partials      398      396       -2     

Impacted Files	Coverage Δ
jupyter_server/base/handlers.py	64.37% <100.00%> (+0.07%)	⬆️
jupyter_server/services/kernels/kernelmanager.py	79.79% <0.00%> (-2.06%)	⬇️

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[havok2063]

@Zsailer Yeah, no problem. I've made the requested changes and did a few checks that allow_origin_pat works as expected.

[Zsailer]

Thanks, @havok2063!

[welcome]

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