# Fix sign character class accidentally matching a pipe `|`

## Problem

`timeparse` accepts a leading pipe `|` as though it were a valid sign and
silently parses the rest of the expression, instead of rejecting the malformed
input with `None`:

```python
>>> from pytimeparse import parse
>>> parse('|5m')
300           # expected None
>>> parse('|1:24')
84            # expected None
```

## Root cause

`pytimeparse/timeparse.py`:

```python
SIGN = r'(?P<sign>[+|-])?'
```

Inside a character class, `|` is a literal character, not the alternation
operator. So `[+|-]` matches `+`, `-`, or `|`. When the input starts with `|`,
the `sign` group captures it, the pipe is stripped from the string, and the
remainder is parsed as if no sign were present (`sign` resolves to `1` because
`'|' != '-'`).

## Fix

Use a character class containing only the two intended sign characters:

```python
SIGN = r'(?P<sign>[+-])?'
```

Now a leading `|` is not consumed as a sign; the pipe stays in the string, no
time-format pattern matches it, and `timeparse` returns `None` as it does for
any other unparseable input. Valid `+`/`-` signs are unaffected.

## Test

Extended `test_timeparse_signs` in `pytimeparse/tests/testtimeparse.py`:

```python
self.assertIsNone(timeparse.timeparse('|5m'))
self.assertIsNone(timeparse.timeparse('| 5m'))
self.assertIsNone(timeparse.timeparse('|1:24'))
```

## Gate

- `python -m pytest pytimeparse/tests/testtimeparse.py` -> 49 passed.

Counterfactual (fix reverted, test kept): fails with
`AssertionError: 300 is not None`.