Fix location of tablespace clauses in schema backend column SQL
Python 3 compat fix for callable()
Fixed 19949 -- Cached template loader now caches TemplateDoesNotExist

Thanks @timgraham and @jdunck for the code reviews and Kronuz for bug
report and initial patch.
Fixed #20961 -- Fixed HttpResponse default empty content

Thanks epandurski at gmail.com for the report.
Fixed #20814 -- Improved model field accessor clash error messages

Thanks shai for the suggestion.
Ignore coverage HTML report output dir in tests.

Added it to the list of dirs ignored by tests/runtests.py.
Made coverage ignore files without associated source code.

This only affests reporting not data collection and allows coverage.py
to succesfully finish the generation of e.g. the HTML report.

Cases of code whose execution data is collected during the run phase but
for which no associated source code files can be found at the reporting
phase include tests with egg files and Python module files created at
test execution-time.
Added further fixes, tests for #19949/f33db5a09a.

Thanks Susan Tan. Refs #19949.