Remove pytz dependency, update minimum python version to 3.9, extend tests

[do3cc]

This is the PR to implement #42
I've reviewed timezone/utils.py and noticed a lot of code isn´t used at all.
Then I added tests for windows and macos to make sure, nothing breaks.
Since nothing broke, I got worried and extended the tests to use timezones as provided by Python 3.9.
Then my windows tests failed and I added the necessary dependency for windows.
Now that I am writing this, I am wondering whether we should depend on tzinfo, for windows, as we don´t use the available timezones anywhere. However, I like that there are tests for that.

[justinmayer]

Thanks for your work on this, Patrick.

@venthur / @hugovk: I am preparing for a trip and could use an extra pair of eyes. Would you mind taking a look and providing any feedback you may have?

[do3cc]

After this I also downloaded pelican and made a test run with my modified branch. No failures. However I saw a few skipped tests due to missing locales.

[hugovk]

Also:

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 5b811c0..d2d7027 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -1,7 +1,7 @@
 ---
 repos:
   - repo: https://github.com/pre-commit/pre-commit-hooks
-    rev: v4.6.0
+    rev: v5.0.0
     hooks:
       - id: check-added-large-files
       - id: check-ast
@@ -15,7 +15,7 @@ repos:
       - id: trailing-whitespace
 
   - repo: https://github.com/asottile/pyupgrade
-    rev: v3.15.2
+    rev: v3.19.1
     hooks:
       - id: pyupgrade
-        args: [--py38-plus]
+        args: [--py39-plus]

[do3cc]

Everything addressed. Thanks for the review

[venthur]

Thanks for your contribution and addressing all requests @do3cc! @justinmayer the PR looks good to go.