Bump isort from 4.3.21 to 5.1.1 in /src/requirements (#588)

Bumps [isort](https://github.com/timothycrosley/isort) from 4.3.21 to 5.1.1.
- [Release notes](https://github.com/timothycrosley/isort/releases)
- [Changelog](https://github.com/timothycrosley/isort/blob/develop/CHANGELOG.md)
- [Commits](https://github.com/timothycrosley/isort/compare/4.3.21...5.1.1)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

src/requirements/dev.txt

@@ -6,6 +6,6 @@ coverage==5.1
 codecov==2.1.7
 black==19.10b0
 pre-commit==2.6.0
-isort==4.3.21
+isort==5.1.1
 hypothesis==5.19.1
 flake8==3.8.2