Relax licence restrictions to support custom licences

This change adds support of use of custom licenses for projects. If the
value specified for `licence` is not in the SPDX License List or
"Proprietary", we treat it as a custom licence withe value (as is) as
the name.

As a side-effect, the validation of licence is no longer performed when
executing `poetry check`.

Resolves: python-poetry/poetry#2020

poetry/core/factory.py

@@ -141,15 +141,6 @@ def validate(
 
         if strict:
             # If strict, check the file more thoroughly
-
-            # Checking license
-            license = config.get("license")
-            if license:
-                try:
-                    license_by_id(license)
-                except ValueError:
-                    result["errors"].append("{} is not a valid license".format(license))
-
             if "dependencies" in config:
                 python_versions = config["dependencies"]["python"]
                 if python_versions == "*":

poetry/core/spdx/__init__.py

@@ -17,7 +17,9 @@ def license_by_id(identifier):
     id = identifier.lower()
 
     if id not in _licenses:
-        raise ValueError("Invalid license id: {}".format(identifier))
+        if not identifier:
+            raise ValueError("A license identifier is required")
+        return License(identifier, identifier, False, False)
 
     return _licenses[id]
 

tests/spdx/test_license.py

@@ -48,3 +48,9 @@ def test_proprietary_license():
     license = license_by_id("Proprietary")
 
     assert "License :: Other/Proprietary License" == license.classifier
+
+
+def test_custom_license():
+    license = license_by_id("Amazon Software License")
+
+    assert "License :: Other/Proprietary License" == license.classifier

tests/spdx/test_main.py

@@ -40,4 +40,13 @@ def test_license_by_id_with_full_name():
 
 def test_license_by_id_invalid():
     with pytest.raises(ValueError):
-        license_by_id("invalid")
+        license_by_id("")
+
+
+def test_license_by_id_custom():
+    license = license_by_id("Custom")
+
+    assert license.id == "Custom"
+    assert license.name == "Custom"
+    assert not license.is_osi_approved
+    assert not license.is_deprecated