Beta package is not compiled properly (soaplib==2.0.0-beta2 -> soaplib==beta2)

Here is verbose output before PR:
```
pip-compile --verbose
===> Collecting source requirements
soaplib==2.0.0-beta2 (from requirements.in:1)

===> Normalizing source requirements
soaplib==2.0.0-beta2 (from requirements.in:1)

===> Resolving full tree
- Finding best package matching ['soaplib==2.0.0-beta2']
  Found best match: beta2 (from PyPI)
- Getting dependencies for soaplib-beta2
  Found: [] (from dependency cache)

===> Pinned spec set resolved
- soaplib==beta2

===> Writing compiled files
requirements.in -> requirements.txt
Dependencies updated.
```

And after PR:

```
===> Pinned spec set resolved
- soaplib==2.0.0-beta2
```