fix: use PEP 508 syntax for credentials-themes requirement

[GlugovGrGlib]

Description

The weekly automated "Upgrade Requirements" workflow has failed on every scheduled run since 2026-02-24 (last success: 2026-02-17, which produced #2959). Each run crashes in make upgrade on the very first pip-compile invocation:

pip._internal.exceptions.InvalidEggFragment: <InvalidEggFragment: invalid-egg-fragment>

The cause is this line in requirements/base.in:

git+https://github.com/openedx/credentials-themes.git@0.5.6#egg=edx_credentials_themes==0.5.6

An #egg= fragment may only contain a bare project name; the ==0.5.6 version specifier was never valid. Older pip only warned about it, but the workflow installs the latest pip, and pip >= 26 turned this long-deprecated pattern into a hard error — so pip-compile crashes at the parsing stage before resolving anything.

This PR switches the line to a PEP 508 direct reference
Verified with pip 26.1.2: the old line reproduces the exact InvalidEggFragment failure from the workflow logs, the new line parses correctly to edx_credentials_themes.

Impact

Once merged, the next scheduled bot run will succeed again and open a fresh requirements-upgrade PR. That will also supersede #2959, which is currently unmergeable because its pinned virtualenv==20.37.0 has since been deleted from PyPI.

Note: the workflow's failure notifications go to a 2U-internal OpsGenie address, which is likely why 6+ months of weekly failures went unnoticed.

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[e0d]

Look good.