Infra: CSS: Show borders around merged cells (GH-4817)

Showing all borders around cells fixes the presentation for "merged" cells
(rowspan/colspan, like in PEP 393).

`<table>` elements already have `border-collapse` set, which means
that adjacent borders are merged.
It's not necessary to set inside borders only on cells & outside ones
on the table.

Author: encukou

pep_sphinx_extensions/pep_theme/static/style.css

@@ -236,7 +236,6 @@ div.table-wrapper {
 table {
     width: 100%;
     border-collapse: collapse;
-    border: 1px solid var(--colour-background-accent-strong);
 }
 table caption {
     margin: 1rem 0 .75rem;
@@ -245,22 +244,16 @@ table thead tr {
     background-color: var(--colour-background-accent-medium);
     color: var(--colour-text-strong);
 }
-table tbody tr {
-    border-top: 1px solid var(--colour-background-accent-strong);
-}
 table th,
 table td {
     text-align: left;
     padding: 0.25rem 0.5rem 0.2rem;
+    border: 1px solid var(--colour-background-accent-strong);
 }
 table.pep-zero-table tr td:nth-child(1),
 table.pep-zero-table tr td:nth-child(2) {
     white-space: nowrap;
 }
-table th + th,
-table td + td {
-    border-left: 1px solid var(--colour-background-accent-strong);
-}
 /* Common column widths for PEP status tables  */
 table.pep-zero-table tr td:nth-child(1) {
     width: 5%;