Update black target version to Python 3.7

Since the minimum required version of Python in the project's
dependencies has been updated to 3.7, it makes sense to bump the black
target version to match.

Author: diazona

pyproject.toml

@@ -6,5 +6,5 @@ build-backend = "setuptools.build_meta"
 write_to = "pytest_localserver/_version.py"
 
 [tool.black]
-target-version = ['py36']
+target-version = ['py37']
 line-length = 120