Need to tighten encoding handling for some types.  #180

>>> P.from_string("pkg:pypi/foo:bar@1.2.3")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/tmp/purl/venv/lib/python3.10/site-packages/packageurl/__init__.py", line 508, in from_string
    raise ValueError(msg)
ValueError: Invalid purl 'pkg:pypi/foo:bar@1.2.3' cannot contain a "user:pass@host:port" URL Authority component: ''.
>>> P.from_string("pkg:maven/com.apache:foo/foobar@1.2.3")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/tmp/purl/venv/lib/python3.10/site-packages/packageurl/__init__.py", line 508, in from_string
    raise ValueError(msg)
ValueError: Invalid purl 'pkg:maven/com.apache:foo/foobar@1.2.3' cannot contain a "user:pass@host:port" URL Authority component: ''.
>>> P.from_string("pkg:maven/com.apache$3Afoo/foobar@1.2.3")
PackageURL(type='maven', namespace='com.apache$3Afoo', name='foobar', version='1.2.3', qualifiers={}, subpath=None)
>>> P.from_string("pkg:maven/com.apache%3Afoo/foobar@1.2.3")
PackageURL(type='maven', namespace='com.apache:foo', name='foobar', version='1.2.3', qualifiers={}, subpath=None)
>>> str(P.from_string("pkg:maven/com.apache%3Afoo/foobar@1.2.3"))
'pkg:maven/c
om.apache:foo/foobar@1.2.3'