rez-pip cannot install astroid 2.2.0 for Python 3.6

I tried installing astroid 2.2.0, which is a dependency of pylint

```
$ rez-pip --install astroid==2.2.0 --python-version=3.6
07:24:47 INFO Trying to use pip from python package
07:24:47 INFO Found pip-20.0.2 inside /home/selecaoone/packages/python/3.6.8/package.py. Will use it with /usr/bin/python3
07:24:47 INFO Installing 'astroid==2.2.0' with pip taken from '/usr/bin/python3'
Collecting astroid==2.2.0
Using cached https://files.pythonhosted.org/packages/31/e9/610c16ecc417c1918915f8f30e54fcfd195788fe450cf8d7d1a061e14628/astroid-2.2.0-py3-none-any.whl
Collecting six (from astroid==2.2.0)
Using cached https://files.pythonhosted.org/packages/65/eb/1f97cb97bfc2390a276969c6fae16075da282f5058082d4cb10c6c5c1dba/six-1.14.0-py2.py3-none-any.whl
Collecting lazy-object-proxy (from astroid==2.2.0)
Using cached https://files.pythonhosted.org/packages/0b/dd/b1e3407e9e6913cf178e506cd0dee818e58694d9a5cd1984e3f6a8b9a10f/lazy_object_proxy-1.4.3-cp36-cp36m-manylinux1_x86_64.whl
Collecting typed-ast<1.3.0; python_version < "3.7" and implementation_name == "cpython" (from astroid==2.2.0)
Using cached https://files.pythonhosted.org/packages/52/6c/57ecbb2ecb80274b7f2b7f95d0937de310aa2e66839d7ae9436881525b23/typed_ast-1.2.0-cp36-cp36m-manylinux1_x86_64.whl
Collecting wrapt (from astroid==2.2.0)
Installing collected packages: six, lazy-object-proxy, typed-ast, wrapt, astroid
Successfully installed astroid-2.2.0 lazy-object-proxy-1.4.3 six-1.14.0 typed-ast-1.2.0 wrapt-1.12.1
WARNING: You are using pip version 19.1.1, however version 20.0.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Traceback (most recent call last):
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/metadata.py", line 732, in __init__
self._data = json.loads(data)
File "/usr/lib64/python3.6/json/__init__.py", line 354, in loads
return _default_decoder.decode(s)
File "/usr/lib64/python3.6/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib64/python3.6/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/home/selecaoone/rez-python-3.6/bin/rez/rez-pip", line 8, in
sys.exit(run_rez_pip())
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/cli/_entry_points.py", line 187, in run_rez_pip
return run("pip")
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/cli/_main.py", line 152, in run
returncode = run_cmd()
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/cli/_main.py", line 144, in run_cmd
return opts.func(opts, opts.parser, extra_arg_groups)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/cli/pip.py", line 67, in command
extra_args=opts.extra)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/pip.py", line 289, in pip_install_package
distributions = list(distribution_path.get_distributions())
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/database.py", line 213, in get_distributions
self._generate_cache()
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/database.py", line 167, in _generate_cache
for dist in self._yield_distributions():
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/database.py", line 148, in _yield_distributions
metadata = Metadata(fileobj=stream, scheme='legacy')
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/metadata.py", line 742, in __init__
scheme=scheme)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/metadata.py", line 285, in __init__
self.read_file(fileobj)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/metadata.py", line 383, in read_file
self.set(field, value)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/metadata.py", line 478, in set
if not scheme.is_valid_constraint_list(value):
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/version.py", line 713, in is_valid_constraint_list
return self.is_valid_matcher('dummy_name (%s)' % s)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/version.py", line 703, in is_valid_matcher
self.matcher(s)
File "/home/selecaoone/rez-python-3.6/lib/python3.6/site-packages/rez/vendor/distlib/version.py", line 116, in __init__
'%r constraints' % op)
ValueError: '.*' not allowed for '>=' constraints
```

astroid 2.2.0 fails but `rez-pip --install astroid==2.1.0 --python-version=3.6` does work. So there may be some change between astroid version 2.1.0 and 2.2.0 which rez-pip does not currently support.

rez: 2.56.1 (tried with both a Python 3.6 and 2.7 install of Rez both fail with the same error)
python: 3.6 / 2.7
os: CentOS-7.7.1908

Reproduce the failure with the shown rez-pip command, then inspect rez/pip.py and the traceback paths in rez/vendor/distlib/metadata.py and version.py. Compare the metadata or constraints handled for astroid 2.1.0 and 2.2.0; done means astroid 2.2.0 installs successfully for the reported Python versions without the distlib exception.

Written by the indexing model from the issue text.