[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/triton/language/random.py

@@ -134,7 +134,7 @@ def pair_uniform_to_normal(u1, u2):
 def randn(seed, offset, n_rounds: tl.constexpr = N_ROUNDS_DEFAULT):
     """
     Given a :code:`seed` scalar and an :code:`offset` block, 
-    returns a block of random :code:`float32` in :math:`\mathcal{N}(0, 1)`
+    returns a block of random :code:`float32` in :math:`\\mathcal{N}(0, 1)`
 
     :param seed: The seed for generating random numbers.
     :param offsets: The offsets to generate random numbers for.
@@ -149,7 +149,7 @@ def randn(seed, offset, n_rounds: tl.constexpr = N_ROUNDS_DEFAULT):
 def randn4x(seed, offset, n_rounds: tl.constexpr = N_ROUNDS_DEFAULT):
     """
     Given a :code:`seed` scalar and an :code:`offset` block,
-    returns a 4 blocks of random :code:`float32` in :math:`\mathcal{N}(0, 1)`
+    returns a 4 blocks of random :code:`float32` in :math:`\\mathcal{N}(0, 1)`
 
     :param seed: The seed for generating random numbers.
     :param offsets: The offsets to generate random numbers for.