Added support for 'py' as the code block type (#47)

People (me included) often use the shorter version of defining python usage within a code block on discord: `py`. This small change should add support for it, the following is also considered a Python code in Discord:

```py
print("Hello world!")
```

Note: Noticed while checking out the code and wanted to help by improving it, didn't test this change but it should work.

coglint/coglint.py

@@ -75,7 +75,7 @@ class CogLint(Cog):
 
         for c in code_blocks:
             is_python, code = c.split(None, 1)
-            is_python = is_python.lower() == "python"
+            is_python = is_python.lower() in ["python", "py"]
             if is_python:  # Then we're in business
                 linted, errors = await self.lint_code(code)
                 linted = linted.getvalue()