dev env: add upper version constraint for isort to keep compatibility with python3.5

Pipfile

@@ -18,10 +18,11 @@ pytest = "*"
 pytest-cov = "*"
 
 # python3.5 compatibility
-# https://github.com/jaraco/zipp/commit/05a3c52b4d41690e0471a2e283cffb500dc0329a
-zipp = "<2"
+isort = "<5"
 # workaround https://github.com/pytest-dev/pytest/issues/3953
 pathlib2 = {version = "*", markers="python_version < '3.6'"}
+# https://github.com/jaraco/zipp/commit/05a3c52b4d41690e0471a2e283cffb500dc0329a
+zipp = "<2"
 
 # python3.8 compatibility
 # workaround https://github.com/WanzenBug/pylint-import-requirements/issues/17

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "48654ce6218ea119cd05b10d6cc14ff27e306885deba781d3265fb0b38d4ad4c"
+            "sha256": "6f1f8ab8743aa4a5323271a7b95dade3884dac8cfaf1271368c63919fca2bea3"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -132,6 +132,7 @@
                 "sha256:54da7e92468955c4fceacd0c86bd0ec997b0e1ee80d97f67c35a78b719dccab1",
                 "sha256:6e811fcb295968434526407adb8796944f1988c5b65e8139058f2014cbe100fd"
             ],
+            "index": "pypi",
             "version": "==4.3.21"
         },
         "lazy-object-proxy": {