Allow for Jupyter Client 7

[jtpio]

References

Fixes #941

Code changes

Update dependency pin on jupyter_client to allow for version 7

User-facing changes

None

Backwards-incompatible changes

?

[github-actions]

Benchmark report

The execution time (in milliseconds) are grouped by test file, test type and browser.
For each case, the following values are computed: min <- [1st quartile - median - 3rd quartile] -> max.

Results table

Test file	basics.ipynb	bqplot.ipynb	dashboard.ipynb	gridspecLayout.ipynb	interactive.ipynb	ipympl.ipynb	ipyvolume.ipynb	multiple_widgets.ipynb	query-strings.ipynb
Render
chromium
actual	3450 <- [3561 - 3587 - 3783] -> 4101	3206 <- [3219 - 3357 - 3376] -> 3441	3395 <- [3443 - 3469 - 3545] -> 3660	4080 <- [4105 - 4107 - 4141] -> 4167	2436 <- [2454 - 2472 - 2477] -> 2586	4200 <- [4337 - 4470 - 4697] -> 6022	10731 <- [18387 - 20174 - 20309] -> 21029	13752 <- [14294 - 14380 - 14455] -> 14469	1673 <- [1690 - 1771 - 1831] -> 1932
expected	3379 <- [3442 - 3517 - 3701] -> 3876	2976 <- [3227 - 3321 - 3421] -> 3604	3608 <- [3623 - 3709 - 3793] -> 3825	4453 <- [4453 - 4523 - 4661] -> 4748	2559 <- [2655 - 2656 - 2660] -> 2674	3982 <- [4079 - 4213 - 4356] -> 4743	12183 <- [18509 - 19553 - 20811] -> 21515	15319 <- [15660 - 15796 - 15912] -> 16056	1517 <- [1920 - 1997 - 2103] -> 2113

❗ Test metadata have changed

--- /dev/fd/63	2021-10-07 09:43:28.726210320 +0000
+++ /dev/fd/62	2021-10-07 09:43:28.726210320 +0000
@@ -42,11 +42,11 @@
       "codename": "Focal Fossa",
       "codepage": "UTF-8",
       "distro": "Ubuntu",
-      "kernel": "5.8.0-1042-azure",
+      "kernel": "5.8.0-1040-azure",
       "logofile": "ubuntu",
       "platform": "linux",
       "release": "20.04.3 LTS",
-      "serial": "6c19bc2f6263432da3d31804b6f46a17",
+      "serial": "cfc067bfcb844f35865e279a1b0e66c5",
       "servicepack": "",
       "uefi": false
     }

[jtpio]

Adding to the 0.3.0 milestone since it's a potential disrupting change.

[jtpio]

Marking as ready for review now that main targets 0.3.0ax

[trungleduc]

Thanks