add upper version constraint for pyyaml to dev/test env to keep compatibility with python3.5 and to install_requires to avoid unexpected breaking changes

Pipfile

@@ -6,6 +6,10 @@ name = "pypi"
 [packages]
 tooncher = {editable = true, path = "."}
 
+# python3.5 compatibility
+# https://github.com/yaml/pyyaml/commit/89f608599d700bf2976bf9307f9f86565f27a80e#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7R295
+pyyaml = "<5.4"
+
 [dev-packages]
 # black requires python>=3.6
 # https://github.com/psf/black/commit/e74117f172e29e8a980e2c9de929ad50d3769150#diff-2eeaed663bd0d25b7e608891384b7298R51

Pipfile.lock

@@ -1,7 +1,7 @@
 {
     "_meta": {
         "hash": {
-            "sha256": "42511b655c01dede2687a6166b274fae08fdd3d4c0647a15896a7fd9c5661332"
+            "sha256": "d6d1c96a9bf968459d8739714460ad758f480ab8a2473fd2288e273bd1070446"
         },
         "pipfile-spec": 6,
         "requires": {
@@ -32,6 +32,7 @@
                 "sha256:cc8955cfbfc7a115fa81d85284ee61147059a753344bc51098f3ccd69b0d7e0c",
                 "sha256:d13155f591e6fcc1ec3b30685d50bf0711574e2c0dfffd7644babf8b5102ca1a"
             ],
+            "index": "pypi",
             "version": "==5.3.1"
         },
         "tooncher": {

setup.py

@@ -27,7 +27,8 @@ setuptools.setup(
         "Topic :: Utilities",
     ],
     entry_points={"console_scripts": ["tooncher = tooncher._cli:main"]},
-    install_requires=["pyyaml"],
+    # pipeline tests againsts pyyaml<5.4
+    install_requires=["pyyaml<6"],
     setup_requires=["setuptools_scm"],
     tests_require=["pytest"],
 )