Add badge for black, py3.7+ and license (sagemath#14)

README.md

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 # Grayskull
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-[![Build Status](https://dev.azure.com/marceloduartetrevisani/Grayskull/_apis/build/status/Tests?branchName=master)](https://dev.azure.com/marceloduartetrevisani/Grayskull/_build/latest?definitionId=4&branchName=master)
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-[![codecov](https://codecov.io/gh/marcelotrevisani/grayskull/branch/master/graph/badge.svg)](https://codecov.io/gh/marcelotrevisani/grayskull)
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+[![Build Status](https://dev.azure.com/marceloduartetrevisani/Grayskull/_apis/build/status/Tests?branchName=master)](https://dev.azure.com/marceloduartetrevisani/Grayskull/_build/latest?definitionId=4&branchName=master) [![codecov](https://codecov.io/gh/marcelotrevisani/grayskull/branch/master/graph/badge.svg)](https://codecov.io/gh/marcelotrevisani/grayskull) [![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black) ![](https://img.shields.io/badge/python-3.7+-blue.svg) ![](https://img.shields.io/github/license/marcelotrevisani/grayskull.svg)
 
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 ![Grayskull](https://comicvine1.cbsistatic.com/uploads/original/4/49448/2661756-castle_grayskull.jpg)