Remove Pipfile

Since it was only used in tox, we can remove it.

Pipfile

@@ -1,17 +0,0 @@
-[[source]]
-url = "https://pypi.org/simple"
-verify_ssl = true
-name = "pypi"
-
-[packages]
-
-[dev-packages]
-mastodon-py = {editable = true, extras = ["tests"], path = "."}
-pytest = "<4"
-pytest-runner = "*"
-pytest-cov = "*"
-vcrpy = "*"
-pytest-vcr = "<1"
-pytest-mock = "*"
-requests-mock = "*"
-