maint: drop Python 3.6

Resolves: #74.

Author: oesteban

setup.cfg

@@ -4,9 +4,10 @@ classifiers =
     Intended Audience :: Science/Research
     Topic :: Scientific/Engineering :: Image Recognition
     License :: OSI Approved :: Apache Software License
-    Programming Language :: Python :: 3.6
     Programming Language :: Python :: 3.7
     Programming Language :: Python :: 3.8
+    Programming Language :: Python :: 3.9
+    Programming Language :: Python :: 3.10
 description = TemplateFlow Python Client - TemplateFlow is the Zone of neuroimaging templates.
 license = Apache-2.0
 license_file = LICENSE
@@ -23,7 +24,7 @@ project_urls =
     Source Code = https://github.com/templateflow/python-client
 
 [options]
-python_requires = >= 3.6
+python_requires = >= 3.7
 setup_requires =
     setuptools >= 45
     setuptools_scm >= 6.2