examples env: add upper version constraint for nbconvert to keep compatibility with python3.5

examples/Pipfile

@@ -11,6 +11,8 @@ notebook = "*"
 # python3.5 compatibility
 # https://github.com/ipython/ipython/commit/0f6d2f42f2a10eeb5f78c57314b2f8cc026aed19#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7L232
 ipython = "<7.10.0"
+# https://github.com/jupyter/nbconvert/commit/e87b847b23e1ae56461f7a7b7d54c44285ce91f1#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7L192
+nbconvert = "<6"
 # https://github.com/ipython/traitlets/commit/8c2554364bd80af917aff1a7725a967ab1203fed
 traitlets = "<5"
 

examples/Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "3185b27b628a56f1353768bf482bf472a6dd1b618de0bde8120f4b06277c2930"
+            "sha256": "fab57e995d6e99e3ddaaa1d6573c637ee8c7fd439751f48d5eb0d47cc737d371"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -252,6 +252,7 @@
                 "sha256:21fb48e700b43e82ba0e3142421a659d7739b65568cc832a13976a77be16b523",
                 "sha256:f0d6ec03875f96df45aa13e21fd9b8450c42d7e1830418cccc008c0df725fcee"
             ],
+            "index": "pypi",
             "markers": "python_version >= '2.7' and python_version not in '3.0, 3.1, 3.2, 3.3, 3.4'",
             "version": "==5.6.1"
         },