Introduce pyproject.toml

Using a more modern setuptools build pipeline, removing warnings in the
generated files:

```
/usr/local/bin/gimme-aws-creds:4: DeprecationWarning:
pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
  __import__('pkg_resources').run_script('gimme-aws-creds==2.7.2', 'gimme-aws-creds')
```

Dockerfile

@@ -9,8 +9,7 @@ RUN apk --update add libgcc
 ENV PACKAGES="gcc musl-dev python3-dev libffi-dev openssl-dev cargo"
 
 RUN apk --update add $PACKAGES \
-    && pip install --upgrade pip setuptools-rust \
-    && python setup.py install \
+    && python -m pip install .\
     && apk del --purge $PACKAGES
 
 ENTRYPOINT ["/usr/local/bin/gimme-aws-creds"]

README.md

@@ -45,7 +45,7 @@ __OR__
 Install the gimme-aws-creds package if you have already cloned the source:
 
 ```bash
-python3 setup.py install
+python -m pip install .
 ```
 
 __OR__

pyproject.toml

@@ -0,0 +1,3 @@
+[build-system]
+requires = ["setuptools", "setuptools-rust", "gimme_aws_creds"]
+build-backend = "setuptools.build_meta"