Drop support for Python 3.6

[leouieda]

Update the python_requires field and metadata in setup.cfg. Bump the
minimum version tested on CI to 3.7.

Fixes #285

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[hugovk]

The CI failure looks temporary.

PR looks good, remember to update the min Python version required in doc/install.rst

Running pyupgrade **/*.py --py37-plus (and Black) only suggests this change:

diff --git a/pooch/hashes.py b/pooch/hashes.py
index 77524bc..d487d5f 100644
--- a/pooch/hashes.py
+++ b/pooch/hashes.py
@@ -208,11 +208,9 @@ def make_registry(directory, output, recursive=True):
         pattern = "*"
 
     files = sorted(
-        [
-            str(path.relative_to(directory))
-            for path in directory.glob(pattern)
-            if path.is_file()
-        ]
+        str(path.relative_to(directory))
+        for path in directory.glob(pattern)
+        if path.is_file()
     )
 
     hashes = [file_hash(str(directory / fname)) for fname in files]

[leouieda]

Thanks @hugovk! I had completely forgotten about that 👍🏽 Removed the list creation as well.

[leouieda]

I think the failures are outages on the Zenodo servers. We don't hit their servers very often so it's OK to restart the jobs once in a while when this fails.