Drop support for Python 3.6 (#309)

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

.github/workflows/test.yml

@@ -39,7 +39,7 @@ jobs:
  - macos
  - windows
  python:
- - "3.6"
+ - "3.7"
  - "3.9"
  dependencies:
  - latest

doc/install.rst

@@ -6,7 +6,7 @@ Installing
 Which Python?
 -------------
 
-You'll need **Python 3.6 or greater**.
+You'll need **Python 3.7 or greater**.
 
 We recommend using the
 `Anaconda Python distribution <https://www.anaconda.com/download>`__

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
@@ -38,7 +37,7 @@ project_urls =
 zip_safe = True
 include_package_data = True
 packages = find:
-python_requires = >=3.6
+python_requires = >=3.7
 install_requires =
  numpy
  pandas