py2deb doesn't handle tilde-equal (~=) version specifications in requirements

[jsbarber]

This is referenced in a couple of comments in issue #8 but that seems to be a different issue. So opening this issue to address this.
I'm specifically talking about #8 (comment):

...
2017-05-22 07:51:22 v2g-dev py2deb.converter[17131] INFO Package pyserial (3.2.1) already converted: /tmp/python3-pyserial_3.2.1_all.deb
2017-05-22 07:51:22 v2g-dev py2deb.cli[17131] ERROR Caught an unhandled exception!
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/dist-packages/py2deb/cli.py", line 192, in main
    archives, relationships = converter.convert(arguments)
  File "/usr/local/lib/python3.5/dist-packages/py2deb/converter.py", line 524, in convert
    archive = package.convert()
  File "/usr/local/lib/python3.5/dist-packages/py2deb/package.py", line 383, in convert
    dependencies = [python_version()] + self.debian_dependencies
  File "/usr/local/lib/python3.5/dist-packages/cached_property.py", line 26, in __get__
    value = obj.__dict__[self.func.__name__] = self.func(obj)
  File "/usr/local/lib/python3.5/dist-packages/py2deb/package.py", line 318, in debian_dependencies
    raise Exception(msg % (constraint, self.python_name))
Exception: Conversion specifier not supported! ('~=' used by Python package uModbus)