Use importlib instead of pkg_resources when possible

[grigaut]

Use of pkg_resources triggers DeprecationWarning and according to https://setuptools.pypa.io/en/latest/pkg_resources.html, pkg_resources should be replaced by importlib.

Minimal steps to reproduce warning:

• Install pytest
• create a file (let's say 'test.py') with :
  import cdsapi
• run pytest test.py

Pytest should raise following warning:
warnings.warn("pkg_resources is deprecated as an API", DeprecationWarning)

[FussyDuck]

All committers have signed the CLA.

[veenstrajelmer]

@grigaut thanks for this PR, it is a step towards fixing #89.

Hierover, I would suggest to replace the current suggestion by something more robust without the need for a try-except statement. The builtin importlib.metadata is available since python 3.8. should work like this:

from importlib.metadata import version

cdsapi_version = version("cdsapi")
self.session.headers = {
        "User-Agent": f"cdsapi/{cdsapi_version}",
        }

I believe this should result in the desired behaviour with less code, but I have not tested it so I might be mistaken.

[EddyCMWF]

Many thanks for you contribution, this has been fixed in #126