Installation instructions on README for pipenv fails

[CorrosiveKid]

Installation instructions the README for pipenv fails with error Could not find a version that matches graphql-core>=3 because pipenv is ignoring pre-release versions.

ERROR: Could not find a version that matches graphql-core>=3
Tried: 0.4.9, 0.4.11, 0.4.12, 0.4.12.1, 0.4.13, 0.4.14, 0.4.15, 0.4.16, 0.4.17, 0.4.18, 0.5, 0.5.1, 0.5.2, 0.5.3, 1.0, 1.0.1, 1.1, 2.0, 2.0, 2.1, 2.1, 2.2, 2.2, 2.2.1, 2.2.1
Skipped pre-versions: 0.1a0, 0.1a1, 0.1a2, 0.1a3, 0.1a4, 0.4.7b0, 0.4.7b1, 0.4.7b2, 0.5b1, 0.5b2, 0.5b3, 1.0.dev20160814231515, 1.0.dev20160822075425, 1.0.dev20160823054952, 1.0.dev20160909030348, 1.0.dev20160909040033, 1.0.dev20160920065529, 1.2.dev20170724044604, 2.0.dev20170801041408, 2.0.dev20170801041408, 2.0.dev20170801051721, 2.0.dev20170801051721, 2.0.dev20171009101843, 2.0.dev20171009101843, 2.1rc0, 2.1rc0, 2.1rc1, 2.1rc1, 2.1rc2, 2.1rc2, 2.1rc3, 2.1rc3, 3.0.0a0, 3.0.0a0, 3.0.0a1, 3.0.0a1, 3.0.0a2, 3.0.0a2

Could be fixed by specifying the LT on the pipenv installation command like so:
pipenv install "graphql-core>=3<4"

[CorrosiveKid]

As @Cito pointed out in #50, the correct way would be as following:
pipenv install --pre "graphql-core>=3a"