Bump virtualenv from 20.0.28 to 20.0.29 in /requirements (#15147)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.0.28 to 20.0.29.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/master/docs/changelog.rst)
- [Commits](https://github.com/pypa/virtualenv/compare/20.0.28...20.0.29)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements/travis_base.txt

@@ -12,9 +12,9 @@ tox==3.18.1 \
     --hash=sha256:5c82e40046a91dbc80b6bd08321b13b4380d8ce3bcb5b62616cb17aaddefbb3a
 
 # Dependencies of Tox
-virtualenv==20.0.28 \
-    --hash=sha256:8f582a030156282a9ee9d319984b759a232b07f86048c1d6a9e394afa44e78c8 \
-    --hash=sha256:688a61d7976d82b92f7906c367e83bb4b3f0af96f8f75bfcd3da95608fe8ac6c
+virtualenv==20.0.29 \
+    --hash=sha256:8aa9c37b082664dbce2236fa420759c02d64109d8e6013593ad13914718a30fd \
+    --hash=sha256:f14a0a98ea4397f0d926cff950361766b6a73cd5975ae7eb259d12919f819a25
 six==1.15.0 \
     --hash=sha256:8b74bedcbbbaca38ff6d7491d76f2b06b3592611af620f8426e82dddb04a5ced \
     --hash=sha256:30639c035cdb23534cd4aa2dd52c3bf48f06e5f4a941509c8bafd8ce11080259