Bump mock from 3.0.5 to 4.0.1 in /requirements (#5947)

Bumps [mock](https://github.com/testing-cabal/mock) from 3.0.5 to 4.0.1.
- [Release notes](https://github.com/testing-cabal/mock/releases)
- [Changelog](https://github.com/testing-cabal/mock/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/testing-cabal/mock/compare/3.0.5...4.0.1)

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

requirements/dev.txt

@@ -18,7 +18,7 @@ django-debug-toolbar==2.0 \
     --hash=sha256:9a23ada2e43cd989195db3c18710b5d7451134a0d48127ab64c1d2ad81700342 \
     --hash=sha256:17c53cd6bf4e7d69902aedf9a1d26c5d3b7369b54c5718744704f27b5a72f35d
 
-mock==3.0.5 --hash=sha256:d157e52d4e5b938c550f39eb2fd15610db062441a9c2747d3dbfa9298211d0f8 --hash=sha256:83657d894c90d5681d62155c82bda9c1187827525880eda8ff5df4ec813437c3
+mock==4.0.1 --hash=sha256:5e48d216809f6f393987ed56920305d8f3c647e6ed35407c1ff2ecb88a9e1151 --hash=sha256:2a572b715f09dd2f0a583d8aeb5bb67d7ed7a8fd31d193cf1227a99c16a67bc3
 
 responses==0.10.9 \
     --hash=sha256:515fd7c024097e5da76e9c4cf719083d181f1c3ddc09c2e0e49284ce863dd263 \