chore: loosen requests-toolbelt dependency (#473)

The "requests-toolbelt" library is only used for "MultipartEncoder"
which has been part of the library since it's initial release.

This patch loosens up the requests-toolbelt dependency to allow for
more flexible co-installability.

pyproject.toml

@@ -42,7 +42,7 @@ sphinx-rtd-theme = {version = "^1.0.0", optional = true}
 readthedocs-sphinx-ext = {version = "^2.1.9", optional = true}
 m2r2 = {version = "^0.3.2", optional = true}
 sphinx-autoapi = {version = "^2.0.0", optional = true}
-requests-toolbelt = ">=1.0.0"
+requests-toolbelt = "*"
 deprecation = ">=2.1.0"
 
 [tool.poetry.extras]