Fix TCL in Windows PyPy tests

[nulano]

Before: https://github.com/python-pillow/Pillow/runs/771448368#step:22:1806

SKIPPED [4] d:\a\Pillow\Pillow\Tests\test_imagetk.py:29: TCL Error: Can't find a usable init.tcl in the following directories: 
    C:/hostedtoolcache/windows/PyPy/3.6.9/x86/lib_pypy/lib/tcl8.5 C:/hostedtoolcache/windows/PyPy/3.6.9/lib/tcl8.5 C:/hostedtoolcache/windows/PyPy/lib/tcl8.5 C:/hostedtoolcache/windows/PyPy/3.6.9/library C:/hostedtoolcache/windows/PyPy/library C:/hostedtoolcache/windows/PyPy/tcl8.5.2/library C:/hostedtoolcache/windows/tcl8.5.2/library

After: https://github.com/nulano/Pillow/runs/774933672?check_suite_focus=true#step:23:1395

Tests/test_imagetk.py::test_kw PASSED                                    [ 93%]
Tests/test_imagetk.py::test_photoimage PASSED                            [ 93%]
Tests/test_imagetk.py::test_photoimage_blank PASSED                      [ 93%]
Tests/test_imagetk.py::test_bitmapimage PASSED                           [ 93%]

---

On Ubuntu there is a different error (below), but adding export DISPLAY=:0.0 only gives a different error (can't connect). There is probably no X server running on Ubuntu.

SKIPPED [4] /home/runner/work/Pillow/Pillow/Tests/test_imagetk.py:29: TCL Error: no display name and no $DISPLAY environment variable

---

Edit: I submitted the issue to pypy, should be fixed in next release, see pypy/pypy#3247. This can be reverted then.

[radarhere]

I've created nulano#6 to update Tcl to 8.6