Tests: prerequisites check improved by using callable.

This is required for more flexible Python version check since ASGI works
for Python 3.5+.  Version check via 'startswith()' function removed as
not consistent.

test/conftest.py

@@ -67,31 +67,39 @@ def pytest_generate_tests(metafunc):
 
     type = cls.application_type
 
-    # take available module from option and generate tests for each version
-
-    for module in cls.prerequisites['modules']:
-        if module in option.available['modules']:
-            prereq_version = cls.prerequisites['modules'][module]
-            available_versions = option.available['modules'][module]
-
-            if prereq_version == 'all':
+    def generate_tests(versions):
         metafunc.fixturenames.append('tmp_ct')
-                metafunc.parametrize('tmp_ct', range(len(available_versions)))
+        metafunc.parametrize('tmp_ct', range(len(versions)))
 
-                for i in range(len(available_versions)):
-                    version = available_versions[i]
+        for i, version in enumerate(versions):
             option.generated_tests[
                 metafunc.function.__name__ + '[{}]'.format(i)
             ] = (type + ' ' + version)
+
+    # take available module from option and generate tests for each version
+
+    for module, prereq_version in cls.prerequisites['modules'].items():
+        if module in option.available['modules']:
+            available_versions = option.available['modules'][module]
+
+            if prereq_version == 'all':
+                generate_tests(available_versions)
+
             elif prereq_version == 'any':
                 option.generated_tests[metafunc.function.__name__] = (
                     type + ' ' + available_versions[0]
                 )
+            elif callable(prereq_version):
+                generate_tests(
+                    list(filter(prereq_version, available_versions))
+                )
+
             else:
-                for version in available_versions:
-                    if version.startswith(prereq_version):
-                        option.generated_tests[metafunc.function.__name__] = (
-                            type + ' ' + version
+                raise ValueError(
+                    """
+Unexpected prerequisite version "%s" for module "%s" in %s.
+'all', 'any' or callable expected."""
+                    % (str(prereq_version), module, str(cls))
                 )