Update crash issue template to use syntax highlighting in code blocks

[brianschubert]

People often forget or don't know how to add syntax highlighting to markdown code blocks (especially for tracebacks!). Let's help them out by pre-filling the template with code blocks that have the right language hints.

[sterliakov]

Good idea, thanks! Is python-traceback tag OK with non-traceback lines? Reports will often have several mypy diagnostics before Traceback (most recent call last:

[brianschubert]

Looks ok to me:

TESTFILES/SCRATCH.py:1: note: Revealed type is "int"
TESTFILES/SCRATCH.py:2: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.18.0+dev.ad76e1628e2d4fe52d89a80d51890bf84afad38f
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
# <snip>
  File "/home/brian/Projects/open-contrib/mypy/mypy/semanal.py", line 2215, in clean_up_bases_and_infer_type_variables
    assert isinstance(node.node, TypeVarLikeExpr)
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: