env: downgrade importlib-metadata to fix pipeline on python3.5

fphammerle/freesurfer-stats@3e663f9

Pipfile

@@ -16,9 +16,12 @@ pytest-cov = "*"
 pytest-timeout = "*"
 
 # python3.5 compatibility
-# https://github.com/python/importlib_metadata/commit/107f9029fd5807c6579b881db19e11a0488f0675
 # fix pluggy/manager.py (v0.13.1) on python<3.8: import importlib_metadata => ModuleNotFoundError
-importlib-metadata = {version = "<3", markers="python_version < '3.8'"}
+# <2 for:
+# > pkg_resources.VersionConflict: (importlib-metadata 2.1.1 (/home/travis/virtualenv/python3.5.10/lib/python3.5/site-packages),
+# > Requirement.parse('importlib-metadata<2,>=0.12; python_version < "3.8"'))
+# <3 for "python_requires" https://github.com/python/importlib_metadata/commit/107f9029fd5807c6579b881db19e11a0488f0675
+importlib-metadata = {version = "<2,<3", markers="python_version < '3.8'"}
 isort = "<5"
 # workaround https://github.com/pytest-dev/pytest/issues/3953
 pathlib2 = {version = "*", markers="python_version < '3.6'"}