This fixes two test failures under Python 3.13:

```
Failed Tests (2):
  FileCheck.py integration tests :: tests/extra_features/pseudo_numeric_variables/01_line_variable_once/test.itest
  FileCheck.py integration tests :: tests/extra_features/pseudo_numeric_variables/02_line_variable_twice/test.itest
```

Both are failing due to:

```
filecheck/filecheck.py:372: DeprecationWarning: 'count' is passed as positional argument
```

Passing `count` as positional argument to `re.sub()` has been deprecated
in Python 3.13: https://docs.python.org/3.13/whatsnew/3.13.html#id9
and produces a `DeprecationWarning` exception.

…hon_313