Upgrade Python to 3.9.1, pip to 20.3.3

[mkoeppe]

This contains the upgrade to Python 3.9.1

To test, use

./configure --without-system-python3 --enable-download-from-upstream-url

Depends on #31050

CC: @slel @jhpalmieri @fchapoton @dimpase @vbraun @kiwifb

Component: packages: standard

Author: Matthias Koeppe

Branch/Commit: 41d7e7a

Reviewer: John Palmieri, Samuel Lelièvre

Issue created by migration from https://trac.sagemath.org/ticket/30589

[mkoeppe]

Dependencies: #30184

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1,3 +1,5 @@
 This contains the experimental upgrade to Python 3.9 for testing ticket #30184 (Support Python 3.9)
 
 It is not intended for merging.
+
+Necessary changes to the library should go on #30184 (Support Python 3.9)

[mkoeppe]

Description changed:

--- 
+++ 
@@ -2,4 +2,10 @@
 
 It is not intended for merging.
 
+To test, use 
+
+```
+./configure --without-system-python3 --enable-download-from-upstream-url
+```
+
 Necessary changes to the library should go on #30184 (Support Python 3.9)

[mkoeppe]

Branch: u/mkoeppe/upgrade-python-3.9

[sagetrac-git]

Commit: e188c5e

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

e188c5e

src/sage/cpython/debugimpl.c: Disable _type_debug for Python 3.9

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. Last 10 new commits:

d3dd329	Merge branch 't/30595/remove_deprecated_sage_libs_ppl' into t/30184/support_python_3_9
68ef4c5	src/sage/cpython/debugimpl.c: Disable _type_debug for Python 3.9
f3cb571	src/sage/all.py: Filter out Python 3.9 deprecation warnings from ast
e16ded2	src/sage/symbolic/ring.pyx (isidentifier): Remove py2-only code, use of deprecated parser module
27a57bc	src/sage/misc/sageinspect.py: Adjust doctest for sage_getsourcelines for Python 3.9 - decorators are now part of the definition
39416fb	src/sage/misc/sageinspect.py (SageArgSpecVisitor): Relax doctest pattern for python 3.9
5548c0d	src/sage/misc/latex.py (has_latex_attr): Relax doctest pattern for python 3.9
4a9362e	src/sage/combinat/root_system/pieri_factors.py: Use sorted in doctest for python 3.9
49eccd9	src/sage/geometry/polyhedron/palp_database.py: Relax doctest pattern for python 3.9
5e89ab2	Merge branch 't/30184/support_python_3_9' into t/30589/upgrade-python-3.9

[sagetrac-git]

Changed commit from e188c5e to 5e89ab2

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

b21ed97	build/pkgs/python3/spkg-configure.m4: Ensure that PYTHON_FOR_VENV is a full path
49aa061	Revert "build/pkgs/python3/spkg-configure.m4: Use 'python >= 3.7'; keep checking for 'python3.8, python3.7'"
a8b77cd	Revert "only test python3, to be 3.7 or 3.8"
9403629	build/bin/sage-system-python: Work around LC_ALL=C
ff0dbc6	build/sage_bootstrap/uncompress/tar_file.py: Fix encoding to utf-8
f05a110	Merge branch 't/30008/after__30053__sphinx_3_1_2_does_not_build_on_ubuntu__trusty_xenial_bionic___debian_jessie__centos_7__again_' into t/30576/public/30576
945c8c5	build/bin/sage-site (--docbuild): Set an English locale suuch as C.utf-8, but not the C locale, to ensure correct operation on Python 3.6
8906897	Merge branch 't/30576/public/30576' into t/30546/build/py3x_conf
60137bc	Merge branch 't/30546/build/py3x_conf' into t/30184/support_python_3_9
316ae6d	Merge branch 't/30184/support_python_3_9' into t/30589/upgrade-python-3.9

[sagetrac-git]

Changed commit from 5e89ab2 to 316ae6d

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. Last 10 new commits:

ca58693	build/pkgs/pillow/spkg-install.in: Install via bdist_wheel
5a747c4	build/bin/sage-pip-{install,uninstall}: Remove pip2 support
9ee2110	build/bin/sage-dist-helpers: Also use $sudo for storing the wheel file
d7aac84	src/doc/en/developer/packaging.rst: Update sdh_... documentation
9b7c7a0	build/bin/sage-pip-{install,uninstall}: Fix typo in comment
4135e8b	build/bin/sage-pip-install: Remove an outdated comment
f2e7075	Merge tag '9.2.beta13' into t/29500/install_all_python_packages_via_pip_wheel__create_pep_503_simple_repository_for_wheels
a2dbb8d	Merge branch 't/29500/install_all_python_packages_via_pip_wheel__create_pep_503_simple_repository_for_wheels' into t/30546/build/py3x_conf
8c756ed	Merge branch 't/30546/build/py3x_conf' into t/30184/support_python_3_9
5cf1baf	Merge branch 't/30184/support_python_3_9' into t/30589/upgrade-python-3.9

[sagetrac-git]

Changed commit from 316ae6d to 5cf1baf

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

f19d9a4	allow absolute names as arguments to --with-python
4f7d588	Merge branch 't/30546/build/py3x_conf' into t/30184/support_python_3_9
b3c1dc1	Merge branch 't/30184/support_python_3_9' into t/30589/upgrade-python-3.9

[sagetrac-git]

Changed commit from 5cf1baf to b3c1dc1

[sagetrac-git]

Changed commit from b3c1dc1 to 6d3a0fa