[FRONTEND] Minor accumulated style and warning fixes (#388)

- Fix some whitespace.
- Make an undeclared dependency on `pytest` explicit.
- Fix deprecated `description-file` use.
- `#ifdef` out a deprecated `PyEval_InitThreads` call.
- Use a slightly different numpy invocation in `test_random.py` to quiet down overflow warnings in tests.
- Fix a deprecated cast in `test_core.py`.
- Suppress a warning about `visit_Constant` in Python 3.9+; we can't migrate yet because it'd break Python 3.6 and 3.7.
- Use chained exceptions for `CompilationError` rather than rolling our own; it makes the error messages nicer.
- Add a `__str__` for `tl.dtype` to make debugging kernels easier; it lets you `print` a dtype to see what type was inferred.
- Fix a few bad escapes.

python/test/unit/language/test_random.py

@@ -74,9 +74,8 @@ class CustomPhilox4x:
         return np.array([ret0, ret1, ret2, ret3], dtype=self._dtype)
 
     def _raise_key(self, key):
-        ret0 = key[0] + self._config.PHILOX_KEY_A
-        ret1 = key[1] + self._config.PHILOX_KEY_B
-        return np.array([ret0, ret1], dtype=self._dtype)
+        pk = [self._config.PHILOX_KEY_A, self._config.PHILOX_KEY_B]
+        return key + np.array(pk, dtype=self._dtype)
 
     def random_raw(self):
         counter = self._counter