Bump isort from 4.3.18 to 4.3.20 (#4971)

Bumps [isort](https://github.com/timothycrosley/isort) from 4.3.18 to 4.3.20.
- [Release notes](https://github.com/timothycrosley/isort/releases)
- [Changelog](https://github.com/timothycrosley/isort/blob/4.3.20/CHANGELOG.md)
- [Commits](https://github.com/timothycrosley/isort/compare/4.3.18...4.3.20)

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

requirements/dev.txt

@@ -35,9 +35,9 @@ flake8==3.7.7 \
     --hash=sha256:a796a115208f5c03b18f332f7c11729812c8c3ded6c46319c59b53efd3819da8 \
     --hash=sha256:859996073f341f2670741b51ec1e67a01da142831aa1fdc6242dbf88dffbe661
 
-isort==4.3.18 \
-    --hash=sha256:1349c6f7c2a0f7539f5f2ace51a9a8e4a37086ce4de6f78f5f53fb041d0a3cd5 \
-    --hash=sha256:f09911f6eb114e5592abe635aded8bf3d2c3144ebcfcaf81ee32e7af7b7d1870
+isort==4.3.20 \
+    --hash=sha256:f57abacd059dc3bd666258d1efb0377510a89777fda3e3274e3c01f7c03ae22d \
+    --hash=sha256:c40744b6bc5162bbb39c1257fe298b7a393861d50978b565f3ccd9cb9de0182a
 
 # Required by pytest
 atomicwrites==1.3.0 \