[PY-27601] Updated fix for "Too few arguments for format string"

There are two important changes:

Based on the implementation at https://github.com/python/cpython/blob/master/Objects/stringlib/unicode_format.h#L82 I have added two new warning messages to PyStringFormatInspection, which detect changes between manual and automatic field numbering.

I have also fixed the way PySubstitutionChunkReference uses myPosition
field to refer to the correct positional argument. It uses the explicitly specified position, if present, or the auto-numbered one otherwise. I have tried to fix all the visible regressions introduced by this change.

python/testData/inspections/PyStringFormatInspection/NewStyleAutomaticAfterManualNumbering.py

@@ -0,0 +1 @@
+<warning descr="Cannot switch from automatic field numbering to manual field specification">'{} {1}'</warning>.format(6, 7)

python/testData/inspections/PyStringFormatInspection/NewStyleManualAfterAutomaticNumbering.py

@@ -0,0 +1 @@
+<warning descr="Cannot switch from manual field specification to automatic field numbering">'{1} {}'</warning>.format(6, 7)

python/testData/inspections/PyStringFormatInspection/NewStylePositionalSubstitutionAfterKeywordSubstitution.py

@@ -1 +1,2 @@
-'{a} {}'.format(6, a=2)
+x = '{a} {}'.format(6, a=2)
+print("{} {other} {}".format("one", "two", other="OTHER"))