Capitalized 'syntax error -> 'Syntax error' for syntax error in type comment messages

[gregorysantosa]

Fixes #15132

1. In my commit I have changed 'syntax error' -> 'Syntax error' for syntax error in type comment within the file 'mypy/message_registry.py. I have also changed all test cases that are related to that stderr message to reflect the proper capitalized error message.

Notes: I did not change the tests for the following test cases despite their messages beginning with lowercase letters because there are no error messages related to printing these messages within 'message_registry.py';
"error: invalid syntax [syntax]"
"can't delete function call"
"can't assign to literal"
"bare * has associated type comment"

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[hauntsaninja]

Thanks, looks great!

Yeah, most syntax errors come from Python's AST parser and it's not worth trying to capitalise them:

>>> import ast; ast.parse("del f()")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/shantanu/.pyenv/versions/3.9.9/lib/python3.9/ast.py", line 50, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 1
    del f()
        ^
SyntaxError: cannot delete function call