Remove redundant Python 2 code

setup.cfg

@@ -107,8 +107,6 @@ exclude_lines =
     pragma: no cover
     # This excludes typing-specific code, which will be validated by mypy anyway.
     if TYPE_CHECKING
-    # Can be set to exclude e.g. `if PY2:` on Python 3
-    ${PIP_CI_COVERAGE_EXCLUDES}
 
 [metadata]
 license_file = LICENSE.txt

tox.ini

@@ -11,11 +11,9 @@ pip = python {toxinidir}/tools/tox_pip.py
 mkdirp = python -c 'import os, sys; os.path.exists(sys.argv[1]) or os.mkdir(sys.argv[1])'
 
 [testenv]
-# Remove USERNAME once we drop PY2.
 passenv =
     CI
     GIT_SSL_CAINFO
-    USERNAME
     HTTP_PROXY
     HTTPS_PROXY
     NO_PROXY
@@ -43,8 +41,6 @@ setenv =
     # Ensure coverage is enabled in child processes in virtual environments
     # since they won't already have been enabled by pytest-cov.
     COVERAGE_PROCESS_START = {toxinidir}/setup.cfg
-    # Used in coverage configuration in setup.cfg.
-    PIP_CI_COVERAGE_EXCLUDES = if PY2
 
 [testenv:docs]
 # Don't skip install here since pip_sphinxext uses pip's internals.