Install PyPi dependency from a PPA build

I am trying the following in my rules file:

override_dh_auto_configure: $(BUILD_FLAGS_FILE)
 python3 -m venv .venv
 rm -rf Build && mkdir -p Build
 . "$${PWD}/.venv/bin/activate" \
   && pip3 --no-cache-dir install "Sphinx" \
   && cd Build \
  && ../bootstrap --prefix=/usr --docdir=/share/doc/cmake --mandir=/share/man \
                   --init=../$(BUILD_FLAGS_FILE) --no-system-libs \
                  --sphinx-man --sphinx-html --sphinx-flags="-D today=\"$(BUILD_DATE)\"" \
                   $(BOOTSTRAP_PARALLEL) --verbose

Locally this works flawless. The Launchpad build machines however fail to install the pip3 package "sphinx":

Collecting Sphinx
  Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fa390fe8978>: Failed to establish a new connection: [Errno -2] Name or service not known',)': /simple/sphinx/
  Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fa390fe8a90>: Failed to establish a new connection: [Errno -2] Name or service not known',)': /simple/sphinx/
  Retrying (Retry(total=2, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fa390fe8b70>: Failed to establish a new connection: [Errno -2] Name or service not known',)': /simple/sphinx/
  Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fa3916556a0>: Failed to establish a new connection: [Errno -2] Name or service not known',)': /simple/sphinx/
  Retrying (Retry(total=0, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7fa391655a58>: Failed to establish a new connection: [Errno -2] Name or service not known',)': /simple/sphinx/
  Could not find a version that satisfies the requirement Sphinx (from versions: )

Can anybody help me with that? Is pypi access maybe actively blocked by launchpad?

Kind Regards
Florian