Expose tox requires via the config object (tox-dev#1919)

Fixes tox-dev#1918

This also fixes broken __str__ of MissingRequirement,
which is never actually used but is helpful when debugging.

docs/changelog/1918.feature.rst

@@ -0,0 +1,2 @@
+The value of the :conf:`requires` configuration option is now exposed via
+the :class:`tox.config.Config` object - by :user:`hroncok`

src/tox/config/__init__.py

@@ -1295,11 +1295,11 @@ def run(name, section, subs, config):
             feedback("--devenv requires only a single -e", sysexit=True)
 
     def handle_provision(self, config, reader):
-        requires_list = reader.getlist("requires")
+        config.requires = reader.getlist("requires")
         config.minversion = reader.getstring("minversion", None)
         config.provision_tox_env = name = reader.getstring("provision_tox_env", ".tox")
         min_version = "tox >= {}".format(config.minversion or Version(tox.__version__).public)
-        deps = self.ensure_requires_satisfied(config, requires_list, min_version)
+        deps = self.ensure_requires_satisfied(config, config.requires, min_version)
         if config.run_provision:
             section_name = "testenv:{}".format(name)
             if section_name not in self._cfg.sections:

tests/unit/session/test_provision.py

@@ -47,6 +47,35 @@ def test_provision_min_version_is_requires(newconfig, next_tox_major):
     assert config.ignore_basepython_conflict is False
 
 
+def test_provision_config_has_minversion_and_requires(newconfig, next_tox_major):
+    with pytest.raises(MissingRequirement) as context:
+        newconfig(
+            [],
+            """\
+            [tox]
+            minversion = {}
+            requires =
+                setuptools > 2
+                pip > 3
+            """.format(
+                next_tox_major,
+            ),
+        )
+    config = context.value.config
+
+    assert config.run_provision is True
+    assert config.minversion == next_tox_major
+    assert config.requires == ["setuptools > 2", "pip > 3"]
+
+
+def test_provision_config_empty_minversion_and_requires(newconfig, next_tox_major):
+    config = newconfig([], "")
+
+    assert config.run_provision is False
+    assert config.minversion is None
+    assert config.requires == []
+
+
 def test_provision_tox_change_name(newconfig):
     config = newconfig(
         [],