Replace pkg_resources (#38)

Importing pkg_resources is slow.

According to setuptools, use of pkg_resources is discouraged in favor of
importlib.metadata

`importlib.metadata` was introduced in Python 3.8 but there exists a
backport (`importlib_metadata`) for older versions of Python.

See: https://setuptools.pypa.io/en/latest/pkg_resources.html

onfido/resource.py

@@ -1,12 +1,15 @@
-import pkg_resources
 import requests
 from .onfido_download import OnfidoDownload
 from .exceptions import error_decorator, OnfidoUnknownError
 from .mimetype import mimetype_from_name
 from .utils import form_data_converter
 
+try:
+    import importlib.metadata as importlib_metadata
+except ImportError:
+    import importlib_metadata
 
-CURRENT_VERSION = pkg_resources.get_distribution("onfido-python").version
+CURRENT_VERSION = importlib_metadata.version("onfido-python")
 
 class Resource:
     def __init__(self, api_token, region, timeout):

pyproject.toml

@@ -17,6 +17,7 @@ packages = [
 [tool.poetry.dependencies]
 python = ">=3.6,<4"
 requests = "^2.24.0"
+importlib-metadata = {version = ">=0.12", markers = "python_version < \"3.8\""}
 
 [tool.poetry.dev-dependencies]
 pytest = "*"