build: clear venv and skip pip self-upgrade to avoid stale pip corruption

[Fector101]

Closes #3339

Problem

run_pymodules_install creates a build venv with python -m venv venv.
If the build directory already exists from a previous failed run, the
venv can contain a mixed pip installation with files from two different
pip versions — pip install -U pip replaces files one by one, and if
the process is interrupted mid-upgrade, the old and new files get
interleaved. On re-run, python -m venv venv happily reuses the
existing directory without cleaning it, so the corrupted state sticks
around permanently.

This showed up as:

ImportError: cannot import name 'BuildDependencyInstallError'
from 'pip._internal.exceptions'

Or, as reported in #3339:

ImportError: cannot import name 'RequirementInformation'
from 'pip._vendor.resolvelib.structs'

Fix

Two changes in pythonforandroid/build.py:

1. --clear flag to python -m venv — wipes the old venv before
   recreating it so stale pip files don't survive between runs.

2. Removed pip install -U pip — the pip that ships with the host
   Python's ensurepip is perfectly capable of installing Cython and
   pure-Python .whl files. Self-upgrading pip adds risk with zero
   benefit.

Testing

• tox -e pep8 — passes
• make test — 460 passed, 2 failed (pre-existing failures in
  test_pythonpackage_basic.py, confirmed against base commit)