Drop support for Python 3.6 (#364)

Remove the compatibility metadata, remove from the CI matrix, bump the
`python_requires` to 3.7+.

.github/workflows/test.yml

@@ -42,7 +42,7 @@ jobs:
           - macos
           - windows
         python:
-          - "3.6"
+          - "3.7"
           - "3.10"
         dependencies:
           - latest

setup.cfg

@@ -23,7 +23,6 @@ classifiers =
     Topic :: Scientific/Engineering
     Topic :: Software Development :: Libraries
     Programming Language :: Python :: 3 :: Only
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
     Programming Language :: Python :: 3.9
@@ -39,7 +38,7 @@ project_urls =
 zip_safe = True
 include_package_data = True
 packages = find:
-python_requires = >=3.6
+python_requires = >=3.7
 install_requires =
     numpy
     scipy