Bump ipykernel from 6.3.1 to 6.4.1

Bumps [ipykernel](https://ipython.org) from 6.3.1 to 6.4.1.

---
updated-dependencies:
- dependency-name: ipykernel
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -16,7 +16,7 @@ fastavro==1.4.4
 fitsio==1.1.5
 gwemopt==0.0.73
 healpy==1.15.0
-ipykernel==6.3.1
+ipykernel==6.4.1
 jupyter==1.0.0
 ligo-gracedb==2.7.6
 lxml==4.6.3

setup.py

@@ -41,7 +41,7 @@
         "fitsio == 1.1.5",
         "gwemopt == 0.0.73",
         "healpy == 1.15.0",
-        "ipykernel == 6.3.1",
+        "ipykernel == 6.4.1",
         "jupyter == 1.0.0",
         "ligo-gracedb == 2.7.6",
         "lxml==4.6.3",