Actually implement the load_grammar properly.

test/test_load_grammar.py

@@ -0,0 +1,31 @@
+import pytest
+from parso.grammar import load_grammar
+from parso import grammar
+
+
+def test_load_inexisting_grammar():
+    # This version shouldn't be out for a while, but if we ever do, wow!
+    with pytest.raises(NotImplementedError):
+        load_grammar('15.8')
+    # The same is true for very old grammars (even though this is probably not
+    # going to be an issue.
+    with pytest.raises(NotImplementedError):
+        load_grammar('1.5')
+
+
+@pytest.mark.parametrize(('string', 'result'), [
+    ('2', 27), ('3', 36), ('1.1', 11), ('1.1.1', 11), ('300.1.31', 3001)
+])
+def test_parse_version(string, result):
+    assert grammar._parse_version(string) == result
+
+
+@pytest.mark.parametrize('string', ['1.', 'a', '#', '1.3.4.5', '1.12'])
+def test_invalid_grammar_version(string):
+    with pytest.raises(ValueError):
+        load_grammar(string)
+
+
+def test_grammar_int_version():
+    with pytest.raises(TypeError):
+        load_grammar(3.2)