RustPython changes for #824

[BenLewis-Seequent]

Split from #824 as requested. These are the changes to RustPython itself to support importing and running unittest.

The changes are:

• Ability to do an absolute import of a non-top-level module. i.e. import unittest.case. This isn't implemented properly but is a hack to get importing of those to happen.
• Add function.call
• Add io.IOBase.flush
• Add skeleton _thread module
• Expose types.MethodType
• Add fake sys.intern and sys.warnoptions
• Make classmethod have a dict by default.

[codecov-io]

Codecov Report

Merging #829 into master will decrease coverage by 0.08%.
The diff coverage is 26%.

@@            Coverage Diff            @@
##           master    #829      +/-   ##
=========================================
- Coverage   63.28%   63.2%   -0.09%     
=========================================
  Files          84      85       +1     
  Lines       13929   13955      +26     
  Branches     3088    3089       +1     
=========================================
+ Hits         8815    8820       +5     
- Misses       3108    3127      +19     
- Partials     2006    2008       +2

Impacted Files	Coverage Δ
vm/src/obj/objclassmethod.rs	90% <ø> (ø)	⬆️
vm/src/stdlib/thread.rs	0% <0%> (ø)
vm/src/vm.rs	71.06% <0%> (-0.49%)	⬇️
vm/src/stdlib/types.rs	0% <0%> (ø)	⬆️
vm/src/pyobject.rs	80.26% <0%> (ø)	⬆️
vm/src/eval.rs	64.28% <0%> (+8.03%)	⬆️
vm/src/stdlib/mod.rs	100% <100%> (ø)	⬆️
vm/src/obj/objobject.rs	75.81% <20%> (-2.04%)	⬇️
vm/src/obj/objfunction.rs	64.15% <25%> (-3.2%)	⬇️
vm/src/sysmodule.rs	65.45% <40%> (-2.55%)	⬇️
... and 5 more

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[cthulahoops]

Looks good, aside from test failure on windows.

[windelbouwman]

Looks good to me too!