doc: update doc/sphinx/requirements.txt

Update the following requirements to their latest version:

* Pygments - syntax highlighting
* pytz     - world timezone definitions
* certifi  - Mozilla's CA bundle

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/sphinx/requirements.txt b/doc/sphinx/requirements.txt
index aed4492..b74661a 100644
--- a/doc/sphinx/requirements.txt
+++ b/doc/sphinx/requirements.txt
@@ -1,6 +1,6 @@
 alabaster==0.7.12
 Babel==2.9.1
-certifi==2022.12.7
+certifi==2023.5.7
 charset-normalizer==2.0.12
 docutils==0.16
 idna==3.3
@@ -8,9 +8,9 @@
 Jinja2==3.0.3
 MarkupSafe==2.1.1
 packaging==21.3
-Pygments==2.11.2
+Pygments==2.15.1
 pyparsing==3.0.7
-pytz==2022.1
+pytz==2023.3
 requests==2.31.0
 six==1.16.0
 snowballstemmer==2.2.0